AU607405B2 - Composite electrodes for use in solid electrolyte devices - Google Patents

Description

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INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: WO 87/ 02715 11/08, HO1M 4/92 Al 27/3 H01M (13) International Publication Date: 7 May 1987 (07.05.87) (21) International Application Number: PCT/AU86/00305 (81) Designated States: AT (European patent), AU, BE (European patent), CH (European patent), DE (Euro- (22) International Filing Date: 15 October 1986 (15.10.86) pean patent), FR (European patent), GB (European patent), IT (European patent), JP, LU (European patent), NL (European patent), SE (European patent), (31) Priority Application Number: PH 3161 US.

(32) Priority Date: 29 October 1985 (29.10.85) Published (33) Priority Country: AU With international search report.

(71) Applicant (for all designated States except US): COM- MONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION [AU/AU]; Limestone Avenue, Campbell, ACT 2601 6 2. /86 (72) Inventor; and Inventor/Applicant (for US only) BADWAL, Sukhvin- t der, Pal, Singh [AU/AU]; 68 Lea Road, Mulgrave, VIC 3170 rjA\ (74) Agents: CORBETT, Terence, Guy et al.; Davies Col- lison, 1 Little Collins Street, Melbourne, VIC 3000 19

(AU).

(54) Title: COMPOSITE ELECTRODES FOR USE IN SOLID ELECTROLYTE DEVICES (57) Abstract A composite electrode material for use in solid electrolyte devices, which comprises a mixture of a noble metal and a semiconducting metal oxide with either electronic (n-type) or hole (p-type) conductivity.

This document contains the amendments rnade under Section 49 and is correct for printitg.

ir Vr 91Y-- i -i PCT/AU86/00305 WO 7/0,2715, COMPOSITE ELECTRODES FOR USE IN SOLID ELECTROLYTE DEVICES This invention relates to composite electrodes which are intended for use in solid electrolyte devices. Such devices may include, for example, oxygen sensors, oxygen pumps, fuel cells, steam electrolysis cells, and electrochemical reactors.

Solid electrolyte devices of interest usually consist of an oxygen ion conducting electrolyte membrane held between two electrodes. The examples of solid electrolyte materials include zirconia, both partially or fully stabilized by the addition of calcia, magnesia, yttria, scandia or one of a number of rare earth oxides, and thoria or ceria doped with calcia or yttria or a suitable rare earth oxide. The electrodes for these devices usually consist of porous coatings of metals such as Pt, Ag, Au, Pd, Ni and Co or metal oxides with-good electronic conductivity.

The electrodes participate in the charge transfer reaction between gaseous oxygen molecules or fuels (such as hydrogen, carbon monoxide or methane) and oxygen ions in the solid electrolyte by donating or V *1 WO 87/02715 PCT/AU86/00305 2 accepting electrons. The electrode may also help to catalyse the reaction. For example Pt, the most commonly used electrode material on solid electrolyte oxygen sensors and oxygen pumps, shows high catalytic activity at temperatures above 600-700C for the 2oxygen charge transfer reaction (02 4e202-) at the electrode/electrolyte interface. The physical and chemical characteristics of the electrode play an important role in determining the speed of response and efficiency of the solid electrolyte devices.

The potentiometric or Nernst sensor consists of an oxygen ion conducting membrane of a solid electrolyte (with negligible electronic conductivity) such as fully or partially stabilized zirconia and two ee rl to 2- S electrodes reversible to 0/O2- redox equilibria. If 2 both electrodes of such a cell are exposed to different oxygen partial pressures an emf is established across' the cell which with respect to air as the reference atmosphere is given by the nernst equation: E(mV) 0.0496 T Log (0.,21/po 2 where p0 2 is the unknown oxygen partial pressure and T 'is the absolute temperature. The emf is measured by making electrical contacts to the electrodes.

Australian patent No. 466,251 describes various geometrically distinct forms of solid electrolyte oxygen sensor. The most commonly-used form is that of i S. i L WO $7/Q2715 PCT/AU86/00305 3 a tube, either open-ended or closed at one end, made entirely from the solid electrolyte. Other designs use the solid electrolyte as a disc or pellet, sealed in one end of a metal or ceramic supporting tube. In all cases the reference environment, which is generally air, is maintained on one side of the tube (commonly on the inside) and.the test environment is exposed to the other side of the tube.

In a potentiometric sensor, the current carrying capabilities of the electrodes are not important although electrodes with high charge transfer rates are required especially for low temperature (below 600 0 C) applications of the sensors. The solid electrolyte oxygen sensors with noble metal electrodes are generally used above 600-700°C. Below these temperatures they suffer from slow response rates, high impedance susceptibility to electrical noise pick-up, large errors and exceptionally high sensitivity to impurities in the flue gases.

Other solid state electrochemical devices such as oxygen pumps, fuel cells, steam electrolysers and electrochemical reactors may consist of a tube of fully or partially stabilized zirconia electrolyte with electrodes coated both on the outside and inside of the tube. A number of such cells may be connected in series and/or parallel to achieve the desired characteristics. For example in an oxygen pump a number of cells may operate in conjunction to increase the yield of.oxygen. In a-fuel cell arrangement a i:r h:(i a i

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air I li_; r L- I- Lr iJ~ i WO 87 5 PCT/AU86/00305 WO 87/02715 4 number of the small cells may be connected in series and parallel to increase the total current and voltage output as maximum theoretical'voltage achievab-le fromf a single cell is around 1.0-1.5 volts. In all these systems the current carrying capacity and hence the overall efficiency of the cell is determined by (i) S the electrode/electrolyte interfacial resistance to charge transfer reaction and (ii) the electrolyte resistance. The interfacial resistance to charge transfer depends mainly on the electrochemical behaviour, and physical and chemical nature of the electrode. Because of the high electrode and electrolyte resistance at low temperature, the cells need to be operated at temperatures in the vicinity of 900-1000C. For optimum efficiency and increased cell life it is essential that these cells be operated at lower temperatures. The voltage losses across the electrolyte can be reduced by the use of thin and mechanically strong electrolyte films. Since conventional metal or metal oxide only electrodes have high electrode resistance at lower temperatures, it is necessary therefore to devise better electrode materials in order to reduce overpotential losses across the electrode/electrolyte interface.

We have found that semiconductor metal oxides S with either electron (n-type) or hole (p-type) conduction when combined-with noble metals such as Pt (referred-to-here-in-after as composite electrodes) are much better electrodes than either metal or metal oxide only electrodes. These electrodes when used in

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w WO8 7/02715 PCT/AU86/00305 oxygen sensors lower their operating temperature to 300°C, well below the 600°C achievable with conventional metal or metal oxide electrodes. In addition when these composite electrodes are used in other solid electrochemical devices, increase their efficiency and enable them to be operated at temperatures lower thlan those achievable with codventional metal or metal oxide only electrodes.

Moreover these composite electrodes have superior physical characteristics such as resistance to grain growth and better adhesion to the electrolyte surface when compared with metal or metal oxide electrodes respectively.

The improved electrochemical behaviour is due to both constituents of the composite electrode participating in dissociation/diffusion/charge transfer reactions near and at the electrode/electrolyte interface.

One objective of the present invention therefore is to provide electrodes: for solid electrolyte oxygen sensors which enable such sensors to generate ideal Nernstian) emfs under oxygen-excess gaseous condition at temperatures substantially below those at which conventional noble metal or metal oxide electrodes begin to show non-ideal behaviour. A further objective is'to provide electrodes which retain their good low temperature behaviour after exposure to-temperatures, as high as 900°C.

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0 In solid electrolyte devices such as oxygen pumps, fuel cells electrochemical reactors and steam electrolysis cells, the low electrode/electrolyte interfacial resistance to charge transfer reactions and, consequently, the current carrying capabilities and current potential characteristics of the electrodes are of utmost importance as distinct from potentiorretric oxygen sensors.

10 A further objective of the present invention is to provide electrode materials for oxygen pumps, fuel cells, electrochemical reactors and steam electrolysers to increase their efficiency, reduce energy losses and increase their useful life time by operation at lower 15 temperatures.

According to the present invention, therefore, there is provided an oxygen ion-conducting solid inorganic electrolyte device said device carrying at least one 20 composite electrode which comprises an electrode material containing a minor proportion by weight of a noble metal and a major proportion of a semi-conducting metal oxide with either electronic (n-type) or hole (p-type) conductivity selected from the oxides of one or more 25 transition metals having atomic numbers 21 30, 39 48 and 72 80, the lanthanides having atomic numbers 57 71, and the actinides having atomic numbers 89 91 and 93 96.

Preferably, the noble metal is platinum, silver, gold, iridium or rhodium or palladium or mixtures or alloys of any two or more of these metals.

The semiconductor metal oxide may be chosen from any suitable oxide which is a good electronic conductor and possesses other required attributes such as thermal

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5 5@ stability and chemical compatibility with the solid electrolyte. Usually the oxide will be selected from semiconducting oxides of one or more of the Transition Metals, Lanthanides or Actinides. In this specification the "Transition Metals" are those having Atomic Nos. 21- 39-48 and 72-80.

In solid electrolyte devices, such as oxygen sensors, oxygen pumps, fuel cells, electrochemical 10 reactors or steam electrolysis cells, the composite electrode material may be provided in the form of a surface layer on a solid electrolyte body.

The said oxide component of the composite electrode 15 may also contain one or more insulator, ionic conductor or other semiconductor phases.

According to one embodiment of the invention, the surface layer may consist of a thin porous coating of a 20 mixture of the noble metal(s) and particles of the semiconducting oxide.

The said surface layer may comprise a thin region on and extending beneath the surface of the solid 25 electrolyte, the said region being enriched in the said composite electrode material.

The invention also includes methods for producing the composite electrode materials of the invention.

The components of the electrode material may be applied to a solid electrolyte surface by any suitable known coating method, such as painting, sputtering, ion implantation, spraying or other in-situ chemical or electrochemical techniques. It is preferred to prepare the semiconducting oxide material before application 901126,tgcO321ettgc65287.1et7 r ii: j.

i r r: I i,: i i j 4 i:: t -8although it is possible (where appropriate) to apply precursors or individual components of the oxide material (or compounds which will produce them) in intimate admixture and to produce the semiconducting metal oxide by sintering the coating. The noble metal is applied with the oxide material (or its component oxides).

Either the elemental metal or a suitable compound which can be heat-decomposed to the metal can be used.

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The electrode materials of this invention permit the construction of solid state electrochemical cells with a 15 low interfacial impedance between the electrode and the solid electrolyte. Similar devices with conventional noble metal or metal oxide only electrodes show a much higher electrode/electrolyte interfacial impedance.

20 The electrodes materials of the present invention also permit the construction of oxygen sensors whose performance under typical air-excess combustion conditions conform to the Nernst equation down to temperatures as low as 300'C. The same sensor with conventional noble metal or metal oxide only electrodes depart significantly from the Nernst equation below 450*C. Sensors equipped with electrodes of the present invention may therefore be used to measure the oxygen potential of gases boiler flue gases, internal combustion engine exhausts) in the temperature range 300°C-700 0 C, where sensors with conventional electrodes require supplementary heating to generate Nernstian emfs.

For gases below 300 0 C, the electrodes of the present invention enable sensors to operate with supplementary heating to bring them to a temperature in the range of 901126,tg032.1etgc65287.1et8 ~-rc.

WO 87/02715. PCT/AU86/00305 300 0 C-400°C, whereas sensors with conventional electrodes are generally heated above 700 0 C. The lower operating temperature reduces the explosion hazard associated with maintaining heated sensors in the flue of a combustion device such as a boiler.

The invention also includes solid electrolyte devices which incorporate a solid electrode in accordance with the invention.

In the following description, reference will be made to the accompanying drawings, in which: Figure 1 shows a longitudinal section of an oxygen sensor embodying the electrodes of the invention; Figure 2 shows one form of a cell for use-in solid-electrochemical devices such as oxygen pumps, fuel cells, electrochemical reactors or steam electrolysers; Figure 3 is'a graph showing plots of the electrodes resistance (Ro) versus composite electrode S[consisting of (U0.38 Sc0.62 )2+x and PtO 2 composition at three different temperatures; Figure 4 is a graph showing a plot of the electrode time constant,To versus composite electrode [consisting of (U0.

3 8 Sc 0 .62)O2+x and PtO2] composition at 600°C;

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j:I WO 87/02715 PCT/AU86/00305 Figure 5 is a graph showing a plot of the electrode resistance (Ro) versus composite electrode [consisting of CrNbO 4 and PtO 2 composition; Figure 6 is a graph showing a plot of the electrode time constant,t versus composite electrode [consisting of CrNbO 4 and PtO 2 composition; Figure 7 is a graph comparing current voltage characteristics of a metal oxide (CrNbO 4 electrode with the composite electrode constituting 75 wt% of this metal oxide 25 wt% PtO 2 Figure 8 is a graph showing the performance of oxygen sensors provided with one composite electrode material of the present invention including comparative results for sensors provided with either individual metal oxide or metal constituting the composite electrode; Figure 9 17 are graphs comparaing the sensor performance of various semiconducting metal oxides with the composite electrodes constituting that metal oxide and Pt (added as PtO 2 and Figure 18 is a graph comparing the individual performances of CrNbO 4 and PtO 2 electrodes with composite electrodes consisting of the two materials in various ratios.

In Figures 8 .to 18, the solid lines represent the theoretical response and the symbols show the measured I' *S l WO 87/P2715, PCT/AU86/00305 11 responses. In the interest of clarity, the vertical scales for Figures 8 to 18 have been offset for each set of data, as shown by the scale bars at the right of each figure.

One form of complete sensor for use in gases, incorporating the electrode materials of the present invention, is shown in Figure 1. A hollow ceramic body 10 is closed at one end by a solid electrolyte disc 11. Electrodes 12 and 13 of the present invention are located at the inner and outer surfaces, respectively, of disc 11. Electrical contact to the electrodes .is made by retal wires 14 and 15. Wire 14 is pressed against electrode 12 by spring loading (not shown), applied by means of insulating tube 16. Tube 16 may also be used to convey a reference gas, e.g., air, to electrode 12. if tube 16 is a multi-bore tube, it may also carry a thermocouple (wires 14 and 17) to determine the temperature of electrolyte disc 11, in which case wire 14 form one leg of the thermocouple.

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i ici i:-a .r, .An alternative to wires 14 and 15 for electrical contact to the electrodes 12 and 13 is to use coatings, of platinum, gold, palladium., silver or their alloys, or of the electrode material itself, on the inside and outside surfaces of ceramic body extending from electrodes 12 and 13 to the open end of the sensor. Such coatings may completely cover surfaces of ceramic body 10, or they may consist of 4 WO 87/02715 PCT/AU86/00305 12 continuous strips covering only part of ceramic body For use in molten metals, outer electrode 13 is not required. Electrical contact must be made to the molten metal in the vicinity of the sensor, using an electrical conductor such as wire 15 attached to the sensor but not direct contact with solid electrolyte disc 11, or using a metallic coating on the outer surface or ceramic body 10 as hitherto described, or using a separate electrical conductor adjacent to the sensor. For measurements over extended periods, it is essential that the external electrical contact not dissolve in, or otherwise be attacked by, the molten metal. If a gaseous, reference, air, is used the internal electrode 12 and electrical contact 14 are required.

A further alternative, for use in either gases or molten metals, is for the solid electrolyte to take the form of a closed-end tube or other similar hollow shape, replacing both disc 11 and ceramic body Another alternative, particularly appropriate when measuring gases, is for.the reference environment, e.g. air, to contact external electrode 13 and for the gas or molten metal under test to occupy the inside of the sensor. .In the case of a gas, it may be conveyed to internal electrode 12 by means of tube 16.

One form of a cell to be used in solid electrochemical devices-other than oxygen sensors p.

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.d- WO 87/Q27151 PCT/AU86/00305 13 shown in Figure 2. A hollow porous ceramic substrate body 20 is coated with a thin layer 21 of the electrodes of the present invention. The impervious electrolyte layer 22 is then formed on this electrode by a suitable combination of ceramic processing techniques. The outer electrode layer 23 of the materials of the present invention is then formed on the outer surface of the electrolyte layer. An alternative to this arrangement is not to use a porous substrate support but to coat electrodes of the present invention both on the inside and outside of a 1 presintered electolyte tube.

r i -I- WO 87/02715 PCT/AU6/00305 The following exampl-es -11--ustrate-the-preparation- of--the--composi-te electrodes of this invention and the behaviour of solid electrolyte cells incorporating such electrodes.

Example 1 In order to show the superior characteristics of composite electrodes of the present invention over individual metal or metal oxides, the materials were selected from a wide spectrum of semiconducting oxides. These include,: different crystal structure, (fluorite, rutile, orthorhombic, hexagonal, monoclinic etc.); (ii) varying degree of nonstoichiometry

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2 0 5 LaNiO 3 and Cr 2 0 3 versus PrO2.y, TbO02y (Pr z Gd1-z) 0 2-x, (Nd 0 9 Sro.l)Co03_ x etc.); (iii) simple transition metal oxides (CoO, NiO, (nO 2 ZnO etc.); (iv) rare earth oxides (CeO2_x, PrO2.y TbO2_y); compounds between two and three metal oxides (CrV0 4 CrNb0 4 LaCr0 3 PrCo0 3 (La 0 8 Sro.

2 Cr03-x, (Nd 0 9 Sro.1)Co03_x etc.); (vi) solid solutions(Uz M 1z)02tx (M Sc, Y, Pr, Dy), (Pr z Gdl.z)0 2 and (vii) materials with n-type (electron conduction) or p-type (hole conduction) conductivity.

The simple transi-tion metal and rare earth oxides were obtained from chemical manufacturing companies. Solid solutions and compounds between two or more metal oxides were prepared by one of the following methods: Scr!u CT 1 ,ft i_ i 1~ -I k tP~ PCT/AU86/00305 WO 87/02715' (i) (ii) (iii) Metal salts-to-give required.composition..were-dissolved -in aqueous media followed by coprecipitation with aqueous ammonia solution. The coprecipitated powder was dried and calcined at high temperatures to complete the reaction.

The second method involved dissolving the metal nitrates (in the desired quantity) in water and carefully drying the solution. This was followed by grinding the dried material and firing at high temperatures.

The transition metal oxides were mixed thoroughly in ethanol, dried and heated at high temperatures. In many cases it was necessary to grind and refire the powder several times to complete the reaction.

The completion of the reaction between metal oxides was detected by taking X-ray diffractograms of the fired powders and comparing the results with the literature data.

Various metal oxides tested are listed in Table 1.

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1 i WO 87/02715 PCTIAt 1 6 j'giln 16 Tabl e 1 Composition, nomenclature and crystal structure of various semiconductor metal oxides tested.

No. Nomenclature ,Composition of metal oxide Crystal structure 1 Al Pr0 2 -y 2 A2 T0- 3 ii CeO2; 6 A6 COO 7 A7 Fe 2 0 3

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3 0 4 8 A8 ZnO 9 A9 MnO 2 A10 SnO 2 11 All W0 3 12 A12 MoO 3 13 A13 CdO 14 A14

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2 0 A15 In 2 0 3 16 A16 CrVO 4 17 A17 CrNb04 SU~~~T~JTE sET PCT/AU86/003 0 WO.87/02715.

18 19 21 22 23 24 26 27 28 29 31 32 33 34 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30.

A31 A32 A33 A34 17 CrUO 4 (ZnO) 0 97 (A1 2 0 3 0 03 (Zn0) 0 *g5 (Zr0 2 Li doped Ni 0 Sn doped In 2 0 3 Nd Co03 La Cr03 LaMn 03 LaNiO 3 La Co03 PrCo0 3 La 2 Ni 04 (NiO) 0 5 (La 2 NiO 4 0 (La 08 Sro.

2 )Cr03x (LaO.9 Sr 0 l)M41O3-x (Nd 09 Sr 0 i)Co0 3 -x.

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0 6) 0 2±x The suibscripts indicate a small departure and indicate a large departure from the ideal oxygen/metal atomi ratio for the oxide in question.

S ~ri-TUT f r t i; WO 87/02715 PCT/AU86/00305 Example 2 Fine pastes of various oxides listed in Table 1 in triethylene glycol were prepared by grinding the powdered oxide with a 25 percent solution of triethylene glycol in ethanol until the ethanol had evaporated. The procedure was repeated several times until a uniform and consistent paste was obtained.

Example 3 A number of pastes of platinum dioxide and (U0.

3 8 Sc0.62) 0 2tx in varying ratios were prepared by the proceudre described in example 2.

Table 2 gives the composite electrode compositions and nomenclature.

Table 2 Composition and Nomenclature of Composite Electrodes Described in Example 3 Nomenclature Electrode composition US1 US2 US3 US4 US6 US7 0 wt% PtO 2 100 15 wt% Pt02 85 25 wt% Pt02 75 35 wt% PtO 2 65 50 wt% Pt02 50 75 wt% Pt02 25 100 wt% Pt02 0 (00.

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PCT IA! rgvinnui~ W0,87/92715, 19 Example 4 A number of pastes of platinum 'dioxide and CrNbO 4 in varying ratios v,,ere prepared by the procedure described in example 2. Table 3 gives the composite electrode composition and nomencature.

Table 3 Composition and Nomenclature of Electrodes Described in Example 4.

Nomenci at ure Electrode composition CN1 ICN2 CN3# C N4 CN5 CN6* t wt% P't0 2 100 10 Wt% Pt0 2 90 25 wt% Pt6 2 75 50 wt% PtO 2 a. 50 75 wt% Pt0 2 25 100 Wt% Pt0 0 B17 in Table 4 US7 in Table 2 CrNbO 4 CrNb0 4 CrMb0 4 Cr~4b0 4 CrNb0 4 CrMb0 4 Same as Same as x F S--3TnuTP 4 7 H 901126,tgcO32ilet,tgc652S7.Iet,6 V. wombod" WO 87/02715 PCT/AU86/0030 WO $7/927 Example Fine pastes of composite electrodes comprising a mixture in triethylene glycol of 25 weight percent platinum dioxide' and 75 iweight percent of the metal oxide were prepared by the procedure described 'in example 2. The compositions of the composite electrodes are given in Tabl e 4.

Table 4 Composition and nomenclature of composite electrodes.

Composite electrode Number- Nomenclature composition 11 12 13 14 5 16 17 18 19 10 21 22 23 24 26 27 28 29 20 31 32 33 34 1 2 3 4 5 6 7 8 9 25 wt% Pt0 2 75 wt% Pr0 2 -y 25 wt% Pt0 2 +75 wt%Th0 2 -y 25 wt% Pt0 2 75 wt% Ce0 2 -x 25 wt% Pt0 2 75 wt% Cr 2 0 3 25 wt% Pt0 2 75 wt% NiO 25 wt% Pt0 2 75 wt% CoO 25 wt% Pt0 2 75 wt% (Fe 2

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4 25 wt% Pt0 2 75 wt% ZnO 25 wt% Pt0 2 75 wt% MnO 2 wt% Pt0 2 t 75 wt% Sn0 2 SUUSTiTU~JT. SHEET

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35 B 33 P CT/A U86/00305 WO $7/02715 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 811 B 12 813 814 B15 B16 B17 818 819 B 20 821 B22 B23 824 B25 826 827 828 829 830 B 31 B32 wt% PtO 2 wt% PtO 2 wt% P'toz wt% PtO 2 wt% Pt0 2 wt% PtO 2 wt% Pt0 2 Wt% Pt0 2 Wt%

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wt% Pt 0 2 Wt% P0 wt% PtO 2 wt% P0 wt% PtO 2 wt% PtO 2 Wt% Pt 02 wt% P0 Wt% PtO 2 Wt% Pt02 Wt% PtO 2 wt% PtO 2 Wt% PtO 2 wt% Pt0 2 Wt% PtO 2 75,wt% W0 3 75 wt% MoO 3 75 wt% CdO 75 wt% V 2 0 75 wt% 1fl 2 03 75 wt% CrVO 4 4- 75 wt% CrNbO 4 75 wt% CrU10 4 75 wt% [(ZnO) 0 97 (A1 2 0 3 0 03

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75 wt% [(ZnO) 0 95 (ZrO 2 0 05 75 wt% (Li doped NiO) 75 wt% (Sn doped Mn 2 0 3 75 wt% (NdCoO 3 75 wt% LaCrO 3 75 wt% LaMnO 3 75 wt% LaNi0 3 75 wt% LaCO 3 75 wt% PrCoO 3 75 wt% La 2 NiO 4 75 wt% r:(NiO) 0 5 (La 2 Ni 4 0 5 75 wt% C(LaO.

8 Sr 0 2 )Crj- 3 +75 wt% [La 0 9 Sr 0 i)Mn0 3 -x) +75 wt% [(Nd 0 9 Sr 0 i)CoO3..xJ 75 wt% [(UO.4 DY0.6) 0 2±tx) 75 wt% [(Prz Gdl..z)O2..XJ 4 wt%. Pt0 2 SUBSTITUTE

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WO 87/02715 PCT/AU86/00305 WO 87/02715 22 Example 6 Several composite electrodes from Tables 2-4 were heated at 600°C for 10-15 hours in air. The X-ray diffractograms of the heated electrodes showed that the platinum dioxide had decomposed to platinum without reacting with the metal oxide. In all the tests performed on composite electrodes to determine their electrode behaviour theywere given preheat treatment at 600 0 C to decompose platinum dioxide to platinum. Although in all the tests, the composite electrodes consisted of platinum and the metal oxide but for convenience they will be referred to as consisting of the metal oxide and platinum dioxide (Tables The final weight ratio of Pt/metal oxide differs only slightly from that PtO 2 /metal oxide, Example 7A The composite electrodes described in example 3 (Table 2) were painted on both flat faces of sintered discs of 7 mol% Y 2 0 3 93 mol ZrO 2 (YSZ7) electrolyte (diameter 9.6-9.7 mm, thickness 2.5 mm and density 95% of theoretical for the electrolyte discs) and heated to 600°C for 15 hours in air. The USN/YSZ7/USN (N 1,2,3,4,5,6 or 7) cells were then subjected to complex impedance measurements over the temperature and frequency ranges of 450-600°C and 0.5 mHz-1 MHz r C- C~ ii

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i_: r WO 87/02715, PCT/AU86/00305 23 respectively. Impedance measurements were made with a solartron 1174 frequency response analyzer. The electrode resistance, (.which determines the rate of the oxygen exchange reaction at the electrode/electrolyte interface) and the time constant, T o which determines the speed of response of the solid electrochemical cells to perturbations such as change in the oxygen partial pressure were determined by the'standard analysis of the impedance data in the complex plane.

In figure 3, the electrode resistance, R o has been plotted against weight percent of platinum dioxide at several temperatures. The graphs show a minimum between 15 and 35 wt% Pt02, The initial decrease of about an order of magnitude ,in the electrode resistance, on addition of a mere 15 wt% PtO 2 is dramatic. For Pt0 2 content above 35 wt% the electrode resistance increased .very rapidly.

SIn figure 4 log r is plotted against weight percent of platinum dioxide at 600 Again this graph shows a minimum round 40wt% Pt dioxide at 600 0 C. Again this graph shows a minimum 3round 40 wt% PtO 2 Example 7B The composite electrodes described in example 4 (Table 3) were :painted on 10 mol% yttria 90.mol% zirconia electrolyte discs and subjectd 't impedance measurements as per example 7A. Figure 5 shows a 1; ii 1 ff~ WO 87/02715 PCT/AU86/00305 i 24 plot of the electrode resistance and Figure 6 a plot of the time constant versus weight percent of platinum dioxide for CNX/YSZ7/CNX (X 1,2,3,4,5 or 6) cells at 600 0 C. These graphs show a minimum in the the electrode resistance and the time constant around 25 wt% Pt02.

The time constant and the electrode resistance of composite electrodes containing 25-40 wt% Pt02 are at least an order of nagnitude lower than those for either constituent of the composite. These results quite clearly show that the oxygen exchange reaction at the compositeelectrode/electrolyte interface is much faster than at either metal/electrolyte or metal oxide/electrolyte interface.. The results also show that both constituent of. the composite electrode participate in the oxygen transfer kinetics.

Such phenomena observed for the materials of the present invention are unique and have not been observed or reported before.

Example 8 In order to determ.ine current voltage characteristics two cells, one provided with metal oxide only (CrNb04) and the second cell with a composite electrode consisting of 75 wt% of CrNbO 4 25 wt% PtO 2 were prepared. by painting the respective electrode on both flat sides of i i :1

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Ai PCT/AU8600305 WO.87/02715 sintered discs of 10 mol% yttria 90 mol% zirconia electrolyte. The temperature of each cell was raised to 700 0 C in air and the cell.' left to equilibrate for several horus with the surroundings before making measurements on that cell. The current voltage characteristics were recorded over several temperatures between 5000 and 700 0 C with a galvanostatic current interruption technique. In this method a constant current was passed through the working electrode-electrolyte-counter electrode until a steady state voltage was reached. The current was then interrupted with a fast electronic switch (switching time 0.1 micro second) and the voltage as a function of time (10 microseconds after the current switch off) was recorded with a transient recorder.

The analysis of the data for each current value gives voltage drop across the solid electrolyte (ohmic drop) and voltage drop across both electrode/electrolyte interfaces (overpotential). In figure 7, the 2 Soverpotential current density (current. nornm.ized to 1 cm 2 of the electrode/electrolyte contact area) relationship for the composite -electrode is 'compared with-the corresponding metal oxide only 5 eectrode. These results clearly show that the overpotential losses at the composite electrodes/electrolyte interface are much lower than those at the corresponding metal oxide only electrode/electro,lyte interface.

These experiments demonstrate the superior electrode properties of composite electrodes over their metal oxide counterparts in appli ations where current carrying capacities of the electrodes are important.

A

t WO 87/02715 PCT/AU86/00305 Example q Oxygen sensors of the type shown in Figure'l were prepared using the electrodes described in Tables 1 to 4. In each case the electrolyte disc used comprised a sintered mixture of 50 weight percent alumina and weight percent of a zirconia-scandia solid solution containing 4.7 mol% scandia. The sensor bodies were prepared by high temperature eutectic welding of solid electrolyte discs on to alumina tubes.

Complete sensors were prepared by painting the electrode under test (Pt, metal oxide or a composite consisting of Pt0 2 and the metal oxide) on both flat sides of the welded electrolyte disc. The sensor assemblies were slowly heated to 600 0 C to burn off the triethylene glycol and in the case of composite electrodes also to decompose Pt0 2 to Pt.

Sensor performance tests were carried out between 300 0 C and 600 0 C. These tests comprised the determination of the cell voltage with air .at both electrodes and with air at the inner 'electrodes and oxygen-nitrogen mixtures (1 to 100 percent oxygen) at the outer electrodes; (ii) the effect on the cell voltage of varying the internal air flow rate by an order of magnitude and (iii) sensor resistance measurements with air at both the electrodes. Most tests were performed at temperature intervals of 25°C during both a heating and a cooling cycle. In all.more than 80 sensors were tested including all the electrodes described in Tables 1-4 to show the superior characteristics .of-composite electrodes ovei it.s individual constituents.

i ii

I

1 4 i 4': i i i_ WO 87/02715 PCT/AU86/00305 27 Table 5 compares the resistance of sensors provided with composite electrodes with those provided with individual metal oxide only electrodes at 600 0 C. Ihese values of resistances also include the electrolyte resistance which (as determined by impedance dispersion analysis of a nunber of cells) is around .6-1.2K ohm at 600C. The resistance of sensors provided with composite electrodes is invariably much lower than those provided with semiconductor metal oxide only electrodes. Because of the much higher resistance of sensors provided with metal oxide only electrodes, they were extremely sensitive to electrical noise pick-up at temperatures below 400-500°C. By contrast sensors with composite electrodes showed no such sensitivity even at temperatures as low as 350*C.

Table The total sensor resistance* (with air at both electrodes) at 600*C The total sensor resistance (k ohm) Metal Oxide with metal oxide only with composite electrodes electrodes consisting of 75 wt% of metal oxide 25 wt% PtO 2 Tb02-y 5.5 5.1 Ce02O x 1918 73.2 Cr203 63.1 1.02 NiO 37.9 4.9 SUBSTITUTE SHEET -L C _L i_ WO 87/02715 PCT/AU86/00305 28 Fe 2 0 3 Fe30 4 238.6 10.1 ZnO 307 li Mn02 1.7 1.8 Sn0 2 439 46.9 W0 3 155 1.11 Mo 3 584 5.70 CdO 178 6.7

V

2 0 5 7.8 4.3 In 2 n 3 70.0 2.1 trVO 4 13.2 1.54 CrNbQ 4 214 0.72 (ZnO) 0 97

(A

2 0 3 0 03 34.0 Li doped NiO 12.9 1.37 Sn doped In 2 0 3 577.7 2.61 La Ni 0 3 25.3 1.72 PrCoO 3 5.07 0.91 (LaO 8 Sr 0 o 2 )Cr0 3 -x 15.1 1.33 (Lao.gSro 1 )Mn0 8.4 1.32 (Ndo. gSro. 1)Co0 3 -x 3.57 1.08 (UO04Pro.6)02±x 2.9 2.1 (Pr 0 7 Gd 0 .3) 0 2-x 14.2 3.8 Electrolyte resistance at 600C is about 0.60-1.2 K ohm.

Resistance too high for accurate measurements.

,~~wsJ3eaT iTUT~; sg-%sE7 L i L .L III- L~ WO 87/92715 ,PCT/AU86/00305 29 All sensor with the oxide only or platinun electrodes gave significant zero errors (with air at both electrodes) and showed large deviations from Nernstian relationship (with air.at the inner and oxygen-nitrogen mixtures at the outer electrode) at temperatures below about 450C. In contrast sensors provided with composite electrodes performed satisfactorily down to.350C or below. Figures 8 to 18 compare the results of several sensors provided with electrodes described in Table 1 and Table 4.

In general, the sensors with oxide only or platinum electrodes exhibited much higher sensitivity to changes in the gas flow rate than those with the metal oxide PtO 2 electrodes.

These results clearly demonstrate the superior low temperature performance of the composite electrodes of the present invention over conventional metal oxide only or porous platinum electrodes on solidJ electrolyte oxygen sensors. In particular, the composite electrode materials enable oxygen sensors to operate reliably at temperatures as low as 300 0 C, well below the limit of similar sensors fitted with porous platinum or metal oxide electrodes.

i i:t WO 87/02715 PCT/AU86/00305 Example In order to compare the response of metal oxide only and platinum electrodes to the metal oxide PtO 2 electrode to sudden changes in the oxygen concentration in the gas stream one flat side of 10 mol% Y 2 0 3 mol% ZrO 2 sintered disc (diameter 9.3 mm thickness 25-3.0 mm) was painted with the metal oxide only or platinum electrode and the other side with the metal oxide PtO2 electrode. Both electrodes of the cell were simultaneously exposed to a sudden change in the oxygen concentration. If both electrodes of the cell responde differently, a voltage signal is developed. The sign and shape of the voltage-time curve provide, information on'the'relative speed of..response of both electrodes. These measurements were made on several couples at temperatures between 300°C and 500°C to show the superior characteristics of composite electrodes overtheir metal oxide only or Pt counterparts. In all cases the composite electrode consisting of a metal oxide Pt02 responded much faster compared to the metal oxide only or porous platinum electrode.

These measurements demonstrate quite clearly that composite electrode have much faster rates for oxygen exchange reaction compared the metal oxide only or platinum electrodes. r-i 1~ j.l.-L32.- Y. I I

Claims (14)

1. An oxygen ion-conducting solid inorganic electrolyte device said device carrying at least one composite electrode which comprises an electrode material containing a minor proportion by weight of a noble metal and a major proportion of a semi-conducting metal oxide with either electronic (n-type) or hole (p-type) conductivity selected from the oxides of one or more transition metals having atomic numbers 10 21 30, 39 48 and 72 80, the lanthanides having atomic numbers 57 71, and the actinides having atomic numbers 89 91 and 93 96.

2. A device as claimed in claim 1 wherein the said 15 noble metal is present in the electrode material in an amount of 15 40% by weight.

3. A device as claimed in Claim 1 or Claim 2, characterized in that the noble metal is platinum, 20 silver, gold, palladium, iridium or rhodium or a mixture or alloy of any two or more of said metals.

4. A device as claimed in any one of Claims 1 to 3, characterized in that the oxide component of the 25 electrode material is a compound or a solid solution between one or more of the composite electrode simple binary metal oxides.

A device as claimed in any one of Claims 1 to 3, characterized in that the oxide component of the electrode material is a mixture of one or more i simple metal oxides, compounds or solid solutions.

6. A device as claimed in any one of Claims 1 to characterized in that the said oxide component of the electrode material also contains one or more I o 901126,tgc032letgc65287.Iet31 -32- insulator, ionic conductor or other semi-conductor phases.

7. A device as claimed in any one of Claims 1 to 6 characterized in that the electrode material is in the form of a surface layer on a body of a solid electrolyte.

8. A device as claimed in Claim 7, characterized in 10 that the surface layer comprises a thin porous coating of a mixture of the noble metal and the semi-conducting oxide.

9. A device as claimed in Claim 7, characterized in 15 that the surface layer comprises a thin region on S and extending beneath the surface of a body of a electrolyte, the said region being enriched in the composite electrode material. 20

10. A method for the manufacture of a device, as claimed *i' in Claim 7 or Claim 8, characterized in that there is applied to or formed upon a body of a solid electrolyte a layer comprising the electrode material.

11. A method as claimed in Claim 10, characterized in that there is applied to a body of a solid electrolyte a layer of a mixture comprising: the semi-conducting metal oxide or a substance or mixture of substance which gives rise to the f said oxide on heating; and the noble metal or a compound which gives rise to the noble metal on heating; 901126,tgc 32ietgc65287.1et32 I- lr: V -I i -33- and the layer is then heated to form the electrode material.

12. A method as claimed in Claim 10, characterized in that the said electrode material is preformed before application to the electrolyte surface.

13. A method for the manufacture of a solid electrolyte device as claimed in Claim 7 or Claim 8, 10 characterized in that a layer of the said electrode :material is applied to or formed upon a porous ceramic substrate, on to which is then applied or formed an electrolyte layer and second electrode material layer respectively.

14. An oxygen ion-conducting solid electrolyte device as claimed in Claim 1, or a method for manufacturing the same substantially as hereinbefore described with reference to the Examples and/or the 20 accompanying drawings. Dated this 26th day of November, 1990 COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCE 25 ORGANISATION By Its Patent Attorneys DAVIES COLLISON AL I, x^ 0 901126,tgcO321ettgc65287.Ietf33 r--ii i,