WO1996009106A1

WO1996009106A1 - Solid-state oxygen ion conductors and oxygen separation device

Info

Publication number: WO1996009106A1
Application number: PCT/NZ1995/000092
Authority: WO
Inventors: Michael Graeme Fee
Original assignee: Industrial Research Limited
Priority date: 1994-09-21
Filing date: 1995-09-21
Publication date: 1996-03-28
Also published as: AU3621395A

Abstract

A material of the formula: Bi8-xPxPba-yQyOb, wherein P and Q are each Ag, Au, Cu, Sb, Sn, Zn, Ni, Tl, or V, or any combination thereof, 2.0 « a « 8.0, 0.0 « x < 4.0, 0.0 « y < 3.5, 17 « b « 19, and either x ¸ 0 or y ¸ 0, or a ¸ 4.5 - 5.5. The invention comprises modified bismuth-lead-oxide compounds having ionic conductivity or mixed ionic and electronic conductivity useful for forming oxygen-permeable membranes or similar in particular for use in oxygen separation devices to separate oxygen from gases containing oxygen such as air.

Description

SOLID-STATE OXYGEN ION CONDUCTORS & OXYGEN SEPARATION DEVICE

FIELD OF INVENTION

The invention comprises modified bismuth-lead-oxide compounds having ionic conductivity or mixed ionic and electronic conductivity useful for forming oxygen-permeable membranes or similar in particular for use in oxygen separation devices to separate oxygen from gases containing oxygen such as air.

BACKGROUND

Solid-state oxygen-ion conductors and mixed ionic/electronic conductors often referred to as solid electrolytes have been studied for a number of years. Although they hold great promise for the development of new technologies in the form of oxygen pumps, fuel cells, electrochemical reactors and similar, their practical application has been limited due to the high operational temperatures required.

The majority of oxygen-ion conductors currently in development for commercial use are based on the fluorite structure. They include ZrO₂ stabilised with Y₂O₃ (YSZ), ThO₂/CeO,/HfOJZrO, solid solutions and d-phase Bi₂O₃ stabilised with Er₂O₃ and Y₂O₃ or SrO. YSZ is chemically very stable and exhibits high ionic conductivity at temperatures in the vicinity of 1000°C. but its conductivity is significantly reduced at lower temperatures. Oxygen pumps, in which a thin oxygen-permeable membrane is used to separate oxygen from other gases, require materials with a very high oxygen-ion conduction. The economics of this technique can be significantly enhanced if the oxygen ion conductors used in them meet two further requirements: they should exhibit mixed ionic/electronic conductivity, ideally with the ionic and electronic components of the conductivity being of similar magnitude, to avoid the need for surface electrodes; and they should be able to be used at temperatures significantly below 1000°C.

In recent years there has been a major ongoing search to discover oxygen-ion conductors which have ionic conductivities similar to that of YSZ, but at significantly lower temperatures. Honnart et al (Solid State Ionics, 9-10, 1981) have reported high ionic conductivity in the bismuth-lead-oxide compound Bi₈Pb₅O₁₇ (BPO) at temperatures above 590°C. However this material does not have a high electronic conductivity and lacks mechanical strength and toughness.

SUMMARY OF INVENTION

The present invention provides modified BPO materials having improved ionic conductivities or ionic and electronic conductivities, and/or improved mechanical strength and toughness. Some BPO materials of the invention are mixed ionic and electronic conductors with high ionic and electronic conductivity and are therefore particularly useful for forming oxygen- permeable membranes or similar and for use in oxygen separation devices to separate oxygen from gases containing oxygen such as air. In broad terms in one aspect the invention comprises materials of formula:

Bi_8.xP_xPb_a._yQ_yO_b where:

P and Q are each Ag, Au, Cu, Sb, Sn, Zn. Ni. Tl or V, or any combination thereof;

2.0 < a < 8.0,

0.0 < x < 4.0,

0.0 < y < 3.5,

17 < b < 19. and either x ≠ O or y ≠ O or a ≠ 4.5-5.5.

One preferred material of the invention is off stoichiometric BPO of formula Bi_gPb_aO_{I7 approx} where 2.0 < a < 8.0, preferably 5.5 < a < 8.0, and more preferably 6.0 < a < 7.0. These materials are believed to in general have higher ionic conductivities than the basic BPO compound.

Another preferred compound of the invention is Ag-substituted BPO of formula Bi_g._xAg_xPb_a..Ag,O_{I7 approx} where 0.0 < x and/or y < 1.0 and 0 < x+y and 5.0 < a < 6.5. and more preferably 0.0 < x and/or y < 0.3 and most preferably 0.0 < x and/or y < 0.1. These materials are believed to have in general higher ionic conductivity than the basic BPO compound and also exhibit significant mixed ionic and electronic conductivity. Another preferred compound of the invention is antimony-substituted BPO of formula Bi₈._xSb_xPb_a._ySb_yO_{I7 approx} where 0.0 < x and/or y < 0.5 and 0 < x+y, and 4.5 < a < 5.5 and more preferably 0.0 < x or y < 0.1. These materials are believed to have improved mechanical toughness and strength.

Another preferred compound of the invention is tin-substituted BPO of formula Bi_g._xSn_xPb₅Sn,O_{17 approλ} where 0.0 < x and/or y < 2.5 and 0 < x+y, and 4.5 < a < 5.5, and more preferably 0.0 < x < 0.5, and y = 0. These materials are all believed to have improved mechanical toughness and strength.

Another preferred compound of the invention is nickel-doped BPO of formula Bi₈._xNi_xPb₅._yNi_yO,_{7 approx} where 0.0 < x and/or y < 4.0 and 0 < x+y, and a is about (5/8)(8-x)+y and more preferably 0.5 < x and/or y < 2.5, and most preferably 1.0 < x and/or y < 2.0. These materials are believed to in general have mixed conductivities and show improved thermal expansion characteristics.

Another preferred compound of the invention is copper-substituted BPO of formula Bi₈Pb_a Cu,O_{I7 appro} where x = 0, and 4.0 < y < 5.5 and a is about (5/8)(8-3x)+y. These materials are believed to have in general higher ionic conductivity than the basic BPO compound and also exhibit significant mixed ionic and electronic conductivity. Note that in certain of the above-described compounds, the formula Bi₈._xP_xPb_a._yQ_yO_b can be simplified. For example: if a = (5/8)(8-x)+y then, by making the substitutions z = x + y and P=Q(=R), the formula can be rewritten as: Bi₈Pb₅RA

where b is approximately 17. This corresponds to stoichiometric BPO doped with the metal R or with the oxide of R. This simpler nomenclature will be used in the description of materials produced by doping with a single metal or metal oxide.

BRIEF DESCRIPTION OF THE FIGURES

This invention will be further described with reference to the accompanying figures, wherein:

Figures la and lb are powder x-ray diffraction patterns for Bi_gPb₅O₁₇ at room temperature (in

the tetragonal β₂ phase) and at 600 °C (in the body centred cubic φ phase) respectively (prior art);

Figure 2 is a room temperature powder x-ray diffraction pattern for the off-stoichiometric Pb- rich material Bi₈Pb_{6 5}O,_{8 5};

Figure 3 is a powder x-ray diffraction pattern for the antimony substituted material

Bi₇ ,Sb_{0 8}Pb₅O₁₇; Figure 4 is a powder x-ray diffraction pattern for the tin substituted material BigSi-o ₅Pb_{4 5}O₁₇;

Figure 5 is a powder x-ray diffraction pattern for silver doped Bi₈Pb₅Ag_{0 15}O₁₇;

Figure 6 is a plot comparing the inverse temperature dependence of the conductivity of the tin substituted materials Bi₇₂Pb₅Sno ₈O_|7 and

with Bi₈Pb₅O_π;

Figure 7 is a plot comparing the inverse temperature dependence of the conductivity of the

silver doped material Bi₈Pb₅Ag_{0 2}O₉ with Bi₈Pb₅O,₇;

Figure 8 is a plot showing the dependence of the electric potential developed across a membrane of the silver doped material Bi_gPb₅Ag₀ ,0₁₇ on the log of the oxygen partial pressure

ratio across the membrane;

Figures 9a and 9b are plots showing the dependence of the mechanical properties of the antimony substituted materials Bi_8.xSb_λPb₅O₁₇ on the dopant concentration x - Figure 9a plots

the modulus of rupture and Figure 9b plots the hardness;

Figure 10 is a schematic diagram of an oxygen separation device utilising an oxygen ion conducting membrane;

Figure 11 is a schematic of an improved oxygen separation device; Figure 12 is a powder x-ray diffraction pattern for the nickel doped material Bi₈Pb₅Ni_{2 54}O_]7;

Figure 13 is a plot showing the inverse temperature dependence of the conductivity of the nickel doped material Bi₈Pb₅Ni_{2 54}O₁₇;

Figure 14 is a plot showing the dependence of the relative ionic (t-) and electronic (t_e) components of the conductivity of nickel doped materials Bi₈Pb₅Ni₂O₁₇ on the dopant concentration z; and

Figure 15 is a plot comparing the thermal expansion behaviour of the nickel doped material

Bi_gPb₅Ni_{2 54}0₁₇ with that of Bi₈Pb₅O₁₇ heated and cooled at 3°C/min. - both materials were prepared by sintering at 610°C and slow cooling in air.

DETAILED DESCRIPTION

Materials of the invention can be formed by -any known method for forming ceramic oxide compounds. Typically precursor materials such as metals, metal oxides, metal carbonates and/or metal nitrates are intimately mixed in stoichiometric quantities and sintered in air or oxygen or oxygen containing atmosphere to form a ceramic. Sintering temperatures should normally be in the range 500 - 650 °C. Alternatively the compounds may be formed by

thermal decomposition of nitrates, acetates and/or citrates or similar - in this method nitrate, acetate and/or citrate salts are dissolved in a polar solvent and the solvent is evaporated to form a dry powder which is then sintered to form the ceramic material. Alternatively again an intimate mixture of metals and/or metal oxides and stoichiometric BPO may be formed and sintered. Again materials may be deposited by chemical vapour deposition onto a permeable substrate. Another technique is to deposit a metallic precursor film onto an appropriate substrate and oxidise the film in situ by sintering in an oxygen-containing atmosphere.

Examples 1-7:

The table below shows a range of modified BPO compounds of the invention, according to

the formula Bi_g.„P.Pb_a.. Q.O_b, or the simplified formula Bi₈Pb₅R₂0_I7, which have been

investigated, by their properties:

Example Formula Composition Description

Bi₈._xSn_xPb >₁ 1₇7 _a a_Dp_Dp_rr_oo_xx x=0.2,0,5,1.5,5.0 Single phase for all compositions. Improved Mechanical Properties

2 Bi₈Pb_J_. Sn. O_{17 ipproλ} y=0.2,0.5,1.5,4.0,8.0 Single phase for all compositions. Improved Mechanical Properties. 3

x=0.3,0.8,1.5,3.0 Single phase for x < 1.5. Improved Mechanical Properties. 4 BigPb₅Cu₇O₁ 1₇7_aa_πp_Dp_rr_oo_xx z=0.11,0.37,0.75,0.84, Mixed phase, mixed ionic/electronic

1.87,2.62,3.75 conductor 5 Bi₈Pb₅Ni_zO_{17 approx} z=0.15,0.51, 1.52,2.54, Single phase, mixed ionic/electronic 5.08,15.2 conductor 6 Bi₈Pb,V_z zO 17approx z=0.1,0.33,0.98 Single phase, high conductivity for x=0.1, mixed phases for x>0.1

7 BigPb₅Zn_zO₁₇ approx z=0.11,0.37,1.1 Single phase, high conductivity. 8 Bi₈Pb₅Ag_zO,_7approx 2=0.003,0.01,0.03,0.1, Single phase, mixed ionic/electronic 0.3,1.0,3.0 conductor.

Each of these compounds was prepared using stoichiometric mixtures of Bi₂O₃. PbO and one or more of Zn, SbO, Sn₂O₃, Ag₂O, Ag, Ni, V₂O₅, and CuO or stoichiometric mixtures of pure Bi₈Pb₅O₁₇ and one of Ag₂O, Ag, Ni, or CuO. The starting materials were ground together with an agate mortar and pestle, pressed into pellets then baked at 500 - 620 °C for between 8 and 16 hours in air. The pellets were reground, re-pelletised and sintered at 500-620°C in air for between 8 and 16 hours. A number of analysis techniques were then used on the materials: Powder x-ray powder diffraction and scanning electron microscopy measurements were made to determine the phase purity of the materials. Thermal expansion, hardness and three-point fracture toughness measurements were made to characterise the physical properties of the materials. Ionic and electronic conductivities were measured by a number of techniques including AC impedance analysis in a temperature controlled cell with platinum or gold electrodes, DC conduction measurements using gold blocking and non blocking electrodes, and oxygen concentration cell measurements.

Sb substituted BPO of formula Bi_8.xSb_xPb₅O_]7, forms a single phase material for low Sb concentrations as shown in Figure 3. It has significantly increased mechanical strength and hardness as antimony substitution increases as shown in Figures 9a and 9b. Sn can be substituted for both Bi and Pb to form a single phase material as evidenced in Figure 4. Sn-doping is beneficial to the mechanical strength of the material and there is no significant drop in the conductivity of materials as shown in Figure 6.

Ag substituted BPO of formula Bi_gPb₅Ag_xO₁₇, formed by grinding stoichiometric BPO with 0.75 micron Ag powder and sintering in air at a temperature between 540 and 565 °C exhibits mixed conductivity. The material is single phase at low doping levels as shown in Figure 5. However the transition to the highly conductive BCC phase on heating, and the delayed transition back to th lower conductivity phase on cooling, occur at reduced temperatures in this material. As little a 0.3% Ag by weight (z = 0.1) is sufficient to reduce the transition temperatures by as much as 10° below that found in undoped BPO as evidenced in Figure 7. The addition of Ag also increases th conductivity of the material in the BCC phase above that found in pure stoichiometric BPO a shown in Figure 7. The Ag-doped BPO is a mixed ionic/electronic conductor as shown by Figur 8, as can be seen by the reduced concentration cell voltage measurements. The mixed ionic/electronic conductivity and reduced operating temperature are properties particularly suited to the requirements of an oxygen separation membrane.

The addition of Ni to BPO at levels of 0.3-5% by weight forms a single phase material as shown by Figure 12, with conductivity similar to that of BPO in the temperature range 590-650°C as shown in Figure 13. This material is a mixed ionic/electronic conductor, as determined by concentration cell measurements and AC/DC conductivity measurements. The mixed ionic/electronic conductivity is suited to the requirements of an oxygen separation membrane. By varying the level of Ni doping in the material, the ratio of ionic to electronic conductivity can be varied as shown in Figure 14. This property is a particularly desirable feature as it allows the ionic/electronic conductivity to be tuned for particular applications. For example, in an oxygen separation membrane the ratio of ionic

to electronic conductivity should be approximately 1 :1. The thermal expansion of Ni-doped material (at doping levels as low as 0.3% by weight) is significantly more linear than that of pure BPO across the temperature range 20-620°C - see Figure 15, and is similar to those of potential support and construction materials such as stainless steel and MgO. This simplifies the task of developing a suitable supported membrane structure which is not over-stressed as a result of thermal cycling.

Example 8:

Pb rich BPO according to the formula Bi_gPb_aO,₇ sintered in air at 610°C, was prepared as described above, with non-stoichiometric starting compositions. As shown in Figure 2 the compound forms a single phase material across a wide compositional range 2.5 < a < 8.0, with the same structure as pure BPO. Across the whole compositional range the ionic conductivity of the material is high. The maximum conductivity for the off-stoichiometric compound Bi₈Pb_{6 5}O_{]8 5} was almost twice that for pure BPO produced with the same sintering conditions.

As stated compounds of the invention having high ionic and electronic conductivity such as Ag and Ni doped BPO may be used to form oxygen permeable membranes. There are a number of

morphologies and methods of construction for oxygen-permeable membranes. For example a solid electrolyte membrane may be free-standing and consist solely of the conductor formed by pressing, rolling, tape-casting of a slurry or slip-casting or other techniques. A solid electrolyte may be mixed with other materials such as ceramics or metals in the form of beads, needles, powders, meshes or other morphologies and formed into a membrane by such techniques. The solid electrolyte may

form a contiguous surface deposited on the surface of a porous or oxygen-permeable supporting substrate, by means such as laser-ablation, evaporative deposition, chemical vapour deposition, slip- spraying or any other technique commonly used to prepare ceramics. The solid electrolyte may b embedded in a porous substrate or mixed with another material which is then further treated to for a porous substrate. The solid electrolyte membrane may be deposited on a prepared porous o oxygen-permeable substrate or it may be pre-formed and then attached to a porous substrate. Th solid electrolyte may also be formed by depositing or embedding or attaching a metallic precurso material onto or into a porous or oxygen-permeable substrate and then oxidising the metalli

precursor in-situ to form the solid electrolyte. Other techniques or methods of construction ar possible so long as the membranes are gas-tight so far as possible. Where the membrane i supported by a substrate the substrate material may be an electrical insulator or alternatively ca itself be an oxygen-ion conductor or an electronic conductor or a mixed ionic/electronic conductor.

Membranes, as described above, may in addition include electronically conducting electrode materials on one or both of their surfaces. The solid electrolyte membranes may be in planar, tubular or corrugated geometry or any other geometry that permits a first gas to contact one side o the membrane and a second gas to contact the other side of the membrane.

Oxygen permeable membranes formed from compounds of the invention may be used in oxygen separation devices. Figure 10 shows a simple oxygen separation device which consists of two chambers 1 and 2 separated by an oxygen-permeable membrane 3 formed as described above. The

inlet chamber, membrane and/or the supply gas are heated to a temperature at which the membrane is highly diffusive to oxygen. The operating temperature is preferably between 500 and 650°C. A first gas such as air or other oxygen containing gas (the supply gas), preferably equal to or greater than 10% oxygen concentration, is supplied to the first chamber 1 (the supply chamber) via inlet 4, as indicated by arrow A, and hence to the first surface of the membrane 3. Oxygen undergoes the reaction

O₂ + 4e^" → 2O²-

at the first surface of the membrane and passes across the membrane in the form of O^2" ions to the second surface of the membrane where it returns to the atmosphere of the second chamber 2 (the yield chamber) as molecular oxygen after the reaction

2O²- - O₂ + 4e

In the case of a mixed ionic/electronic conductor such as the BPO compounds of the invention the transport of electrons to support the ionisation/deionisation process can be via the membrane's internal electronic conductivity or by means of an electronic conductor mixed into the membrane or via a conductive support structure, or externally by means of surface electrodes and an external conductive pathway.

A second gas containing a higher relative concentration of oxygen (the yield gas) can be pumped from the second chamber 2 via outlet 5, as indicated by arrow B. A pressure difference is maintained between the two chambers such that the oxygen partial pressure of the supply gas is greater than the oxygen partial pressure of the yield gas. Oxygen depleted gas exits the supply chamber via outlet 6, as indicated by arrow C.

A limitation in any practical oxygen separation device utilising oxygen-ion conducting membranes results from the competing aims of using the thinnest possible membrane, in order to minimise the resistance of the membrane to ionic conductivity, and maintaining the largest

possible oxygen partial pressure differential across the membrane, in order to maximise the chemical potential which drives the oxygen across the membrane. For example, if air is the supply gas and pure oxygen is pumped from the other side of the membrane, a pressure ratio of 5: 1 is required before oxygen will flow across the membrane. If the supply gas is at one atmosphere a certain thickness of membrane will be required in order to withstand the pressure differential. If air is supplied at higher pressures, or the yield chamber is pumped to lower pressures, a thicker, stronger membrane will be required - negating the advantages of a higher oxygen partial pressure differential. However, if a diluting gas is supplied to the yield chamber at sufficient rate and pressure, the total pressures across the membrane can be equalised while a large oxygen partial pressure is maintained. A thinner membrane may be utilised without the risk of rupture. Requirements for the diluent gas are that it should be non- reactive with oxygen at the temperatures and pressures found in the separation device and that it should be simple to separate it from the yield oxygen. Examples of the diluting gas include, but are not limited to, water and carbon-dioxide. The pressure differential across the membrane may be minimised by controlling the pressure of the supply gas or the diluting gas, or by limiting the rate at which gas is pumped from the yield chamber or by use of a one way valve which allows the contents of the yield chamber to leak across to the supply chamber. Figure 17 shows this arrangement. The same reference numbers indicate the same parts as in Figure 16 except that the device also includes an inlet 7 through which the dilutant gas enters the yield chamber, as indicated by arrow D and an optional one-way pressure relief system 8, connecting the two chambers which allows oxygen and the dilutant gas to flow from the yield chamber to the supply chamber.

The foregoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof.

Claims

A material of formula:

Bi_{8 x}P-Pb_a._yQ_yO_b

wherein:

P and Q are each Ag, Au, Cu, Sb, Sn, Zn, Ni, Tl, or V, or any combination thereof,

2.0 < a < 8.0,

0.0 < x < 4.0,

0.0 < y < 3.5,

17 < b < 19, and

either x ≠ 0 or y ≠ 0, or a ≠ 4.5 - 5.5.

2. A material according to claim 1 wherein 2.0 < a < 4.5 or 5.5 < a < 8.0 and x and y = 0.

3. A material according to claim 2 wherein 5.5 < a < 8.0.

4. A material according to claim 2 wherein 6.0 < a < 7.0.

5. A material according to claim 1 wherein P and/or Q is Sb, and 0.0 < x < 0.5 and 0.0 < y

< 0.5 and 0 < x+y.

6. A material according to claim 5 wherein 4.5 < a < 5.5, and 0.0 < y < 0.1.

7. A material according to claim 1 wherein P and/or Q are Sn, 4.5 < a < 5.5, 0.0 < x < 2.5, and 0.0 < y < 2.5 and 0 < x+y.

8. A material according to claim 7 wherein 4.5 < a < 5.5, 0.0 < x < 0.5, and y = 0.0.

9. A material according to claim 7 wherein 4.5 < a < 5.5, x = 0, 0.0 < y < 0.5, and x = 0.0.

10. A material according to claim 1 wherein P and/or Q are Ag, 5.0 < a < 6.5, 0.0 ≤ x ≤ 1.0, and 0.0 < y < 1.0 and 0 < x+y.

11. A material according to claim 10 wherein 0.0 < x < 0.3, and 0.0 < y < 0.3.

12. A material according to claim 10 wherein 0.0 < x < 0.1, and 0.0 < y < 0.1.

13. A material according to claim 10 wherein a is about (5/8)(8-x)+y.

14. A material according to claim 11 wherein a is about (5/8)(8-x)+y.

15. A material according to claim 12 wherein a is about (5/8)(8-x)+y.

16. A material according to claim 1 wherein P and/or Q are Ni, 0.0 < x < 4.0, and 0 < y < 4.0 and 0 < x+y.

17. A material according to claim 16 wherein 0.5 < x < 2.5 and 0.5 < y < 2.5.

18. A material according to claim 17 wherein 1.0 < x < 2.0 and 1.0 < y < 2.0.

19. A material according to claim 16 wherein a is about (5/8)(8-x)+y.

20. A material according to claim 17 wherein a is about (5/8)(8-x)+y.

21. A material according to claim 18 wherein a is about (5/8)(8-x)+y.

22. A material according to claim 1 wherein Q is Cu, x = 0.0 and 4.0 < y < 5.5.

23. A compound according to claim 22 wherein a = (5/8)(8-3x).

24. An ionic and/or electronic conducting membrane comprising a compound according to any one of claims 1 to 23.

25. A membrane according to claim 24 comprising said compound deposited onto, or attached to the surface of a porous supporting substrate, or embedded or impregnated into a porous substrate or mixed with another material which has been further treated to form a porous substrate.

26. An oxygen separation apparatus comprising two chambers separated by a membran according to either of claims 24 and 25, means to supply an oxygen containing gas to one chambe and means to remove a second gas containing a higher concentration of oxygen from the othe chamber, with a pressure difference maintained between the two chambers such that the oxyge partial pressure of the first gas is greater than the oxygen partial pressure of the second gas.

27. An oxygen separation apparatus according to claim 26 including means to supply a diluen gas to said other chamber at such a pressure that the oxygen partial pressure of the first gas i greater than the oxygen partial pressure of the second gas, but the total pressure difference between the two chambers is reduced.