Electrochemical energy conversion devices and cells, and positive electrode-side materials for them

Abstract

An electrochemical energy conversion device 10 comprising a stack of solid oxide electrochemical cells 12 alternating with gas separators 14, 16, wherein scavenger material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the positive electrode-side of the cell 12, the adjacent gas separator 14 and any other structure of the device 10 forming a gas chamber 64 between the cell and the gas separator. The invention also extends to the treated cell 12.

Claims

1. An electrochemical energy conversion device comprising a stack of solid oxide electrochemical cells alternating with gas separators, wherein each electrochemical cell comprises a layer of solid oxide electrolyte, a negative electrode-side structure on one side of the electrolyte layer and comprising one or more porous layers including a functional layer of negative electrode material having an interface with the one side of the electrolyte layer, and a positive electrode-side structure on the opposite side of the electrolyte layer and comprising one or more porous layers including a layer of positive electrode material having an interface with the opposite side of the electrolyte layer, wherein said electrochemical cell and a first of the gas separators on the negative electrode side of the electrochemical cell at least partly form therebetween a negative electrode-side chamber and said electrochemical cell and a second of the gas separators on the positive electrode side of the electrochemical cell at least partly form therebetween a positive electrode-side chamber, and wherein scavenger material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the positive electrode-side structure, the second gas separator and any other structure of the electrochemical energy conversion device forming the positive electrode-side chamber, the scavenger material being accessible to poisons in the atmosphere in the positive electrode-side chamber during use of the electrochemical energy conversion device and being more reactive with the poisons than is the positive electrode material, and wherein if the scavenger material is provided in the layer of positive electrode material there is no scavenger material present at the interface of that layer with the electrolyte layer.

2. An electrochemical energy conversion device according to claim 1, wherein said any other structure forming the positive electrode-side chamber comprises one or more of a separate compliant layer and a separate conducting layer between the second gas separator and the positive electrode-side structure.

3. An electrochemical energy conversion device according to claim 1, wherein the scavenger material is provided in a scavenger coating on one or more of the positive electrode-side structure, the second gas separator and said any other structure forming the positive electrode-side chamber.

4. An electrochemical energy conversion device according to claim 3, wherein the second gas separator comprises a dense substrate and, a protective coating on a side of the substrate facing the positive electrode-side chamber and in contact with the substrate and/or one or more porous layers on a side of the substrate facing the positive electrode-side chamber, and wherein the scavenger coating is provided on the protective coating and/or in at least one of the one or more porous layers of the second gas separator.

5. An electrochemical energy conversion device according to claim 3, wherein the scavenger coating is discontinuous.

6. An electrochemical energy conversion device according to claim 3, wherein the scavenger coating has a thickness of about 0.01 to 250 μm.

7. An electrochemical energy conversion device according to claim 3, wherein the scavenger material in any one porous layer or in the coating layer is provided at a level in the range of about 0.1 to 65 vol % of the total solid content of the layer or coating.

8. An electrochemical energy conversion device according to claim 1, wherein the scavenger material is dispersed in at least one of the one or more porous layers of the positive electrode-side structure.

9. An electrochemical energy conversion device according to claim 8, wherein the one or more porous layers of the positive electrode-side structure comprises, in addition to the layer of positive electrode material, a positive electrode-side layer of electrical contact material and the scavenger material is more reactive with the poisons than is the contact material.

10. An electrochemical energy conversion device according to claim 8, wherein the one or more porous layers of the positive electrode-side structure comprises, in addition to the layer of positive electrode material, a positive electrode-side layer of shield material and the scavenger material is more reactive with the poisons than is the shield material.

11. An electrochemical energy conversion device according to claim 8, wherein the layer of positive electrode material is a functional layer and the one or more porous layers of the positive electrode-side structure comprises, in addition to said functional layer, a substrate layer and the scavenger material is more reactive with the poisons than is the material of the substrate layer.

12. An electrochemical energy conversion device according to claim 1, wherein the scavenger material comprises free oxide selected from one or more of SrO, CaO, BaO, MgO, Na.sub.2O and K.sub.2O.

13. An electrochemical energy conversion device according to claim 1, wherein chemically unbound material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the negative electrode-side structure, the first gas separator and any other structure of the electrochemical energy conversion device forming the negative electrode-side chamber, the chemically unbound material acting to reduce degradation of electrochemical performance on the negative electrode side of the electrochemical energy conversion device during use of the device, and wherein if the chemically unbound material is provided in the functional layer of negative electrode material there is no chemically unbound material present at the interface of that layer with the electrolyte layer.

14. An electrochemical energy conversion cell comprising a layer of solid oxide electrolyte, a negative electrode-side structure on one side of the electrolyte layer and comprising one or more porous layers including a functional layer of negative electrode material having an interface with the one side of the electrolyte layer, and a positive electrode-side structure on the opposite side of the electrolyte layer and comprising one or more porous layers including a layer of positive electrode material having an interface with the opposite side of the electrolyte layer, wherein scavenger material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on the positive electrode-side structure and is accessible to poisons in atmosphere contacting the positive electrode-side structure during use of the electrochemical energy conversion cell and being more reactive with the poisons than is the positive electrode material, and wherein if the scavenger material is provided in the layer of positive electrode material there is no scavenger material present at the interface of that layer with the electrolyte layer.

15. An electrochemical energy conversion cell according to claim 14, wherein the scavenger material is provided in a discontinuous scavenger coating on the positive electrode-side structure.

16. An electrochemical energy conversion cell according to claim 15, wherein the scavenger coating has a thickness of about 0.01 to 250 μm.

17. An electrochemical energy conversion cell according to claim 14, wherein the scavenger material is dispersed in at least one of the one or more porous layers of the positive electrode-side structure.

18. An electrochemical energy conversion cell according to claim 17, wherein the one or more porous layers of the positive electrode-side structure comprises, in addition to the layer of positive electrode material, a positive electrode-side layer of electrical contact material and the scavenger material is more reactive with the poisons than is the contact material.

19. An electrochemical energy conversion cell according to claim 17, wherein the one or more porous layers of the positive electrode-side structure comprises, in addition to the layer of positive electrode material, a positive electrode-side layer of shield material and the scavenger material is more reactive with the poisons than is the shield material.

20. An electrochemical energy conversion cell according to claim 17, wherein the layer of positive electrode material is a functional layer and the one or more porous layers of the positive electrode-side structure comprises, in addition to said functional layer, a substrate layer and the scavenger material is more reactive with the poisons than is the material of the substrate layer.

21. An electrochemical energy conversion cell according to claim 14, wherein the scavenger material in any one porous layer is provided at a level in the range of about 0.1 to 65 vol % of the total solid content of the layer.

22. An electrochemical energy conversion cell according to claim 14, wherein the scavenger material comprises free oxide selected from one or more of SrO, CaO, BaO, MgO, Na.sub.2O and K.sub.2O.

23. An electrochemical energy conversion cell according to claim 14, wherein chemically unbound material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on the negative electrode-side structure and acts to reduce degradation of electrochemical performance on the negative electrode side of the electrochemical energy conversion cell during use of the cell, and wherein if the chemically unbound material is provided in the functional layer of negative electrode material there is no chemically unbound material present at the interface of that layer with the electrolyte layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of an electrochemical energy conversion device in accordance with the invention and test results associated with the various embodiments will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 is a sectional view (not to scale) of a typical fuel cell assembly configuration of the type used to test the invention;

(3) FIG. 2 is a graph of the effect on degradation of the cell assembly of FIG. 1 of introducing free strontium oxide to a negative electrode side cell contact layer;

(4) FIG. 3 is a graph of the effect on degradation of the cell assembly of FIG. 1 of introducing free strontium oxide to a positive electrode side cell contact layer;

(5) FIG. 4 is a graph of the effect on degradation of the cell assembly of FIG. 1 of introducing free strontium oxide to a positive electrode side cell contact layer as well as in both the contact layer and a shield layer;

(6) FIG. 5 is a graph similar to FIGS. 2 to 4 but comparing the effect of providing strontium oxide in various locations on the positive electrode side only, on the negative electrode side only, and on both electrode sides;

(7) FIG. 6 is a graph similar to FIG. 5, but showing the effect on degradation of providing strontium oxide on both positive and negative electrode sides over a longer period that in FIG. 5;

(8) FIG. 7 is a graph of the effect on degradation of the cell assembly of FIG. 1 of introducing free strontium oxide to individual locations of the assembly;

(9) FIG. 8 is a graph similar to FIG. 7, but also showing the overall effect on degradation of providing strontium oxide on numerous locations of the assembly;

(10) FIG. 9 is a graph showing the effect on degradation of different amounts of strontium oxide in various layers of the cell assembly;

(11) FIG. 10 is a graph contrasting the effect on degradation of the cell assembly of FIG. 1 of introducing free oxide to various locations of the cell in different forms;

(12) FIG. 11 is a graph showing the long-term effect on degradation of the cell assembly of FIG. 1 of introducing free strontium oxide to individual locations of the assembly; and

(13) FIG. 12 is a graph contrasting the effect on degradation of different free oxide precursors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(14) FIG. 1 is sectional view illustrating a planar fuel cell assembly 10 of the type used to test the present invention. The figure is not to scale and is provided in the form shown for ease of illustration. The assembly 10 comprises a fuel cell unit 12 between opposed interconnect plates 14 and 16. In a commercial fuel cell device, multiples of these assemblies 10 would be stacked on top of each other, with each pair of abutting interconnector plates 14 and 16 being formed as a single plate. As illustrated, the interconnect plate 14 is a positive electrode- or cathode-side interconnect, while the interconnect plate 16 is a negative electrode- or anode-side interconnect. They are formed of chromium-containing high temperature resistant ferritic steel such as Crofer 22H, Crofer 22APU and ZMG 232L.

(15) Between the interconnect plates 14 and 16, the fuel cell unit 12 comprises a dense electrolyte layer 18 of 8YSZ having a thickness in the range of 5 to 20 μm, for example 10 μm, with a doped ceria barrier layer 20 on the cathode side. The barrier layer 20 prevents reactions between the electrolyte layer 18 and certain cathode layers. Depending on the combination of the electrolyte and cathode materials, the barrier layer may not be necessary. If it is provided, it may be a mixed phase ceria zirconia layer as described in WO2010/040182 and may have a thickness in the range of 0.5 to 1.5 μm.

(16) The cathode layer 22 formed on the electrolyte barrier layer 20 is a porous perovskite such as LSCF and has a thickness in the range of 20 to 30 μm. A cathode shield layer 24 is provided on the cathode layer 22, followed by a cathode cell contact layer 26, both formed of LSCo with the contact layer being more porous than the shield layer, which may in turn be more porous than or of similar porosity to the cathode layer. The shield layer has a high degree of tortuosity relative to the contact layer 26 and a thickness of about 20 μm, and is designed with a large surface area to reduce the likelihood of poisons on the cathode side of the fuel cell unit reaching the cathode layer 12 by virtue of the strontium bound in the perovskite structure reacting with those poisons in the pores of the shield layer. As will be appreciated, the shield layer 24 may be redundant in view of the provision of unbound scavenger material on the cathode side.

(17) The cathode cell contact layer 26 has a thickness of about 125 μm and provides an electrically conductive layer between the interconnect plate 14 and the cathode layer 22.

(18) The cathode interconnect plate 14 is provided with grooves or channels 28 for gaseous oxidant, usually air, supply and the removal of gases on the cathode side (in fuel cell mode). Between the grooves or channels 28 peaks or lands 30 are defined which form contact faces with the cathode cell contact layer 26. To enhance the electrical contact between the cathode interconnect plate 14 and the cathode cell contact layer 26, a cathode interconnect plate contact layer 32 is provided on the lands 30 to directly contact the cathode cell contact layer 26. The cathode plate contact layer 32 is also formed of LSCo and has a similar porosity to the contact layer 26. It may have a thickness of 75 to 125 μm.

(19) Also on the cathode side, a barrier coating 34 is provided across the entire surface 36 of the interconnect plate 14 exposed to the oxidant in use of the cell assembly, between the surface 36 and the contact layer 32. The barrier coating 34 is intended to prevent the release of chromium species from the interconnect plate 14, and may be a spinel layer as described in WO96/28855 having a thickness of 15 to 30 μm.

(20) On the anode side, an anode functional layer 38 having a degree of porosity is provided on the opposite side of the electrolyte layer 18 to the cathode layer 22. It is formed of a Ni/8YSZ cermet having a thickness of 10 to 12 μm.

(21) A porous anode substrate layer 40 of Ni/3YSZ cermet having a thickness of 180 to 200 μm is provided on the opposite side of the anode functional layer 38 to the electrolyte layer 18, and has a greater porosity than the functional layer. The substrate layer 40 acts as a structural support layer for all of the other layers of the fuel cell unit 12.

(22) A anode cell contact layer 42 is provided on the substrate layer 40 on the opposite side to the functional layer 38 to enhance the electrical connection between the anode substrate layer 40 and the anode interconnect plate 16. It is formed of porous metallic nickel, generally more porous than the substrate layer 40, and has a thickness of about 30 μm.

(23) As on the cathode side, the anode interconnect plate 16 is provided with grooves or channels 44 for the delivery of fuel gas to the anode side of the fuel cell unit 12 and removal of reacted fuel (in fuel cell mode). Between the grooves or channels 44, peaks or lands 46 are defined, and the same porous nickel material is provided as an anode interconnect plate contact layer 48 on them. The contact layer 48 may have a thickness of about 100 μm.

(24) To improve electrical conductivity between the interconnect plate 16 and its contact layer 48, a dense layer of nickel 50 is formed on the side 52 of the plate exposed to fuel. The dense nickel coating may have a thickness of 15 to 45 μm and extends across the lands 46 and channels 44.

(25) Most if not all of these layers of the fuel cell assembly 10 are known in the prior art and do not require describing further. However, briefly, the dense electrolyte layer 18 may be made by tape casting particulate 8YSZ slurry and firing it. The electrolyte barrier layer 20 is formed as described in WO2010/040182. The cathode layer 22 is formed by screen printing an ink made with LSCF perovskite material and binder, onto the barrier layer 20 and firing it. The cathode shield layer 24 and contact layer 26 are formed by screen printing an ink comprising LSCo perovskite material and binder, as well as a pore former such as carbon, polymer beads, corn starch, high molecular weight binders or graphite in the case of the contact layer. The shield layer 24 is screen printed onto the cathode layer 22 and the contact layer 26 is screen printed onto the shield layer 24. After screen printing the layers are fired.

(26) The cathode interconnect spinel barrier coating 34 may be formed as described in WO96/28855, while the cathode interconnect contact layer 32 is identical to the cathode cell contact layer 26 but screen printed onto the barrier coating 34. After screen printing the contact layer 32 is fired.

(27) The anode substrate layer 40 is formed first by tape casting a slurry of NiO, 3YSZ, pore former selected from those described above, dispersant and solvent. During firing, the pore former, binder and dispersant burn off, leaving pores and a substrate structure of NiO and 3YSZ. After pre-sintering of the assembly, the NiO in the substrate reduces to Ni to produce the porous Ni/3YSZ substrate structure.

(28) The anode functional layer 38 is tape cast as a slurry of NiO, 8YSZ, binder, dispersant and solvent. During firing, the binder, dispersant and solvent burn off leaving a smaller degree of porosity than in the substrate layer 40. After pre-sintering, the NiO in the substrate reduces to Ni to provide the functional layer of Ni/8YSZ cermet. Porosity in the functional layer arises from the volume change occurring during the NiO.fwdarw.Ni conversion. The electrolyte layer 18 is tape cast onto the functional layer 38.

(29) On the opposite side of the substrate layer 40 the anode cell contact layer 42 is formed by screen printing an ink consisting of Ni, pore former selected from those described above and binder. During the initial heating of the cell assembly 10 (part of the pre-sintering procedure), the binder and pore former bum off, leaving NiO which is subsequently reduced to porous Ni.

(30) The tape cast cell layers may be formed on a preceding layer or one or more may be formed separately and laminated.

(31) The same screen printing ink and procedure are used for the contact layer 48 on the anode interconnect plate 16, while the dense contact layer 50 is formed first by thermally spraying metal powder onto the face 52 so that it is formed in the grooves 44 as well as on the lands 46.

(32) To complete the cell assembly 10 (prior to pre-sintering) the cathode side of the assembly must be sealed from the anode side, and both must be sealed from external atmosphere. To do this, a series of glass seals 54, 56 and 58 and a cover plate 60 are used. The various cathode side layers 22, 24 and 26 of the cell 12 do not extend to the edge of the electrolyte layer 18, and the glass seal is formed on the electrolyte barrier layer 20 as an annulus that extends entirely around the cathode layer 22. The cover plate 60 is formed of the same ferritic steel as the interconnect plates 14 and 16 and is an annulus that is seated on the glass seal 54 and extends outwardly therefrom. Towards its outer periphery 62, the cover plate 60 is also supported on the glass seal 56, which is itself an annulus that is supported on the anode interconnect plate 16 outwardly of the fuel cell unit 12 and at least the anode interconnect plate contact layer 48. The glass seal 58 is also an annulus that is supported on the cover plate 62 and extends to the cathode interconnect plate 14 outwardly of the cathode side of the fuel cell unit 12 and the cathode plate barrier coating 34.

(33) A positive electrode or cathode side oxidant and exhaust chamber 64 is formed between the electrolyte barrier layer 20, the glass seal 54, the cover plate 60, the glass seal 58, the cathode interconnect plate 14 and/or the cathode plate barrier coating 34, with the porous cathode layer 22, cathode shield layer 24, cathode cell contact layer 26 and cathode side interconnect plate contact layer 32 being an integral part of the cathode side chamber 64.

(34) Similarly, a negative electrode or anode side chamber 66 is formed between the electrolyte layer 18, the glass seal 54, the cover plate 62, the glass seal 56, the anode side interconnect plate 16 and/or the dense anode interconnect layer 50, with the anode functional layer 38, the anode porous substrate layer 40, the anode cell contact layer 42 and the anode interconnect plate contact layer 48 being an integral part of the anode side chamber 66.

(35) It will be appreciated that at least one inlet to and at least one outlet from each chamber 64 and 66 must be provided to supply oxidant to and remove oxidant exhaust from the cathode side chamber 64 and supply fuel gas and remove fuel exhaust from the fuel side chamber 66 (in fuel cell mode). These have not been shown in FIG. 1 merely for clarity of the section.

(36) As described, the cell assembly 10 formed the basis of the tests represented as “no Sr” in FIGS. 2 to 8 and 10. It will be appreciated from these graphs of electrical output degradation (measured as a percentage variation relative to an output at about 0 hours against time, where a positive degradation means the electrical output is decreasing) that the cell assembly as described suffers from electrical output degradation for at least one of the reasons described herein.

(37) It will be understood that this degradation occurs even though, for example, at least the cathode shield layer 24 and the cathode plate barrier coating 34 as well as the anode plate dense contact layer 50 are designed to alleviate the degradation.

(38) The applicant has found, though, that the provision of free alkaline earth metal oxygen containing compounds and/or free alkali metal oxygen containing compounds on or in one or both of the positive and negative electrode-side chambers can substantially alleviate the electrical performance degradation. If the fuel cell assembly is to be used for internally reforming hydrocarbon fuel gas, such as natural gas, to hydrogen, alkali metal oxygen containing compounds should be avoided on the negative electrode or anode side since these compounds tend to have a detrimental effect on the reforming reaction.

(39) Generally the oxygen-containing compound will be an oxide, but due to the reactivity of these oxides they need to be added as a precursor. Depending on the thermal stability of the precursor, some precursor material may exist as free oxygen-containing compound alongside the free or unbound oxide. The preferred oxide is SrO (conveniently referred to as “Sr” in FIGS. 2 to 9), but other oxides in the two Groups are known to perform very similarly, particularly CaO, BaO, MgO, Na.sub.2O and K.sub.2O.

(40) The oxygen-containing compound can be provided in or on any one of various components of one or both of the cathode and anode chambers 64 and 66. FIGS. 2 to 9 and 11 illustrate the provision of free SrO, and any residual strontium nitrate precursor, in various of those locations identified in the Figures as positions 1-7. The location of these positions relative to the cell assembly 10 of FIG. 1 is identified in Table 1.

(41) FIG. 10 also illustrates the provision of SrO from strontium nitrate precursor at positions 2 to 6 and the description below for this provision in relation to FIG. 2 to applies to FIG. 10 also. However, FIG. 10 contrasts this provision with providing the free SrO from strontium carbonate and free CaO from calcium carbonate, each at the same locations, positions 2 to 6.

(42) FIG. 12 contrasts the effect on degradation of using different strontium and calcium precursor materials and forms.

(43) TABLE-US-00001 TABLE 1 SrO Position Description of Location 1 Free SrO coating on the anode interconnect plate 16 between the dense contact layer 50 and the porous nickel contact layer 48. 2 Free SrO dispersed in the anode interconnect plate porous contact layer 48. 3 Free SrO dispersed in the anode cell porous nickel contact layer 42. 4 Free SrO dispersed in the cathode shield layer 24. 5 Free SrO dispersed in the cathode cell porous contact layer 26. 6 Free SrO dispersed in the cathode interconnect contact layer 32. 7 Free SrO coating on the cathode interconnect plate 14 between the cathode interconnect plate barrier coating 34 and the cathode interconnect plate porous contact layer 32.

(44) In positions 1 and 7, the free SrO coating or wash coat is applied to the entire extent of the respective dense layer 50 and 34, including in the respective grooves or channels 44 and 28, not just beneath the respective interconnect plate porous contact layer 48 or 32.

(45) The same strontium solution is used for both the coatings at positions 1 and 7 and for dispersing the strontium nitrate precursor in the layers at positions 2 to 6. The Sr(NO.sub.3).sub.2 is converted to SrO on heating in air.

(46) The strontium solution is made up of strontium nitrate, water and a dispersant. Initially, strontium nitrate is weighed, followed by the addition of a required amount of water. The strontium nitrate is dissolved in the water by heating the mixture in a water bath in a temperature range of 40-70° C., while being stirred. A dispersant is added to the solution to prevent the strontium nitrate from recrystallising at temperatures less than 15° C. and to assist dispersing the strontium nitrate in the inks used for incorporating the strontium into the layer materials for positions 2 to 6. The concentration, of strontium nitrate solution is slightly lower than the saturation level at normal temperature and pressure, to avoid the recrystallisation problem.

(47) For the porous nickel contact layers 48 and 42 at positions 2 and 3, the strontium is dispersed in the following way. The required quantities of nickel powder, pore former and binder are weighed and mixed in a high shear mixer. Once the mixture is homogenised, the required quantity of the strontium nitrate solution is added and the new mixture is homogenised again in the high shear mixer to produce an ink suitable for screen printing of the layers.

(48) For the porous perovskite contact layers 26 and 32 at positions 5 and 6, the required quantities of lanthanum strontium cobaltite (LSCo) powder and binder are mixed by hand until they are blended together. The blended mixture is then triple roll milled for a number of passes before the pore former and additional binder are added to the triple-rolled mixture and homogenised in a high shear mixer. Once the mixture is homogenised, the required quantity of the strontium nitrate solution is added and homogenised again in the high shear mixer to produce an ink suitable for screen printing of the layers.

(49) The screen printing ink preparation for the perovskite cathode shield layer 24 at position 4 is prepared in exactly the same way as the cathode side porous contact layer inks, except that no pore former is added to the mixture.

(50) For the screen printing inks, the strontium may be added to a level where in the fired product the free strontium oxide and any residual precursor strontium nitrate are present at a total level of from 0.1 to 65 vol % relative to the total solids content of the layer, more preferably from 1 to 25 vol %. While there may be advantages to providing levels of the free strontium material above 25 vol % up to the maximum indicated of 65 vol %, doing so may lead to difficulties in maintaining the stability of the screen printing inks, and it is for this reason that 25 vol % is the preferred maximum. Levels of strontium oxide tested in the cathode side contact layers and shield layer have been from 2.8-13.2 wt %, while the corresponding range for the anode side contact layers is 0.6-13.2 wt %. Free strontium nitrate has also been added to the anode substrate layer 40, at a level of 0.64 wt %.

(51) Table 2 sets out Examples of compositions for the strontium nitrate solution, the cathode side porous contact layer inks, the cathode shield layer ink and the anode side porous contact layer inks. In Table 2, no ranges are given for the amount of LSCo, pore former and nickel as the LSCo/pore former and Ni/pore former ratios were maintained constant.

(52) Some commercial names are referred to in Table 2 (and in Tables 6 to 8), and these are explained in Table 3, along with their source.

(53) The strontium nitrate solution is applied to the cathode and anode interconnect plates 14 and 16 by spraying the solution onto the respective faces 36 and 52, over the respective dense layers 34 and 50, but excluding the areas contacted by the glass seals 58 and 56, respectively. The required weight of strontium nitrate is achieved by controlling the number of spray passes, to provide coating thicknesses in the range 0.01 to 250 μm, preferably 0.01 to 50 μm.

(54) TABLE-US-00002 TABLE 2 Sr(NO.sub.3).sub.2 Solution Range Material Current Weight, g with min. AMP95 with max. AMP95 Scavenger precursor Sr(NO.sub.3).sub.2 36 36.0% 36.0% Medium Water 54.8 64.0% 44.0% Dispersant AMP95 9.2 0.0% 20.0% Material Current Weight, g Preferred Range, g Broadest Range, g Positive side Porous Layer ink Conducting Phase LSCo 32.9 32.9 32.9 Binder 1 Cerdec 80683 18.7 10-25 1-25 Binder 2 Cerdec 80858 18.7 10-25 1-25 Pore-former Graphite 16.0 16.0 16.0 Scavenger Sr(NO.sub.3).sub.2 solution 13.7 1.32-42.0 0.132-232   Positive side Shield Layer ink Conducting Phase LSCo 48.9 48.9 48.9 Binder 1 Cerdec 80683 16.3 10-25 1-25 Binder 2 Cerdec 80858 16.3 10-25 1-25 Scavenger Sr(NO.sub.3).sub.2 solution 18.5 1.97-62.4 0.197-345   Negative side Porous Layer ink Conducting Phase LSCo 35.6 35.6 35.6 Binder 1 Cerdec 80683 18.3 10-25 1-25 Binder 2 Cerdec 80858 18.3 10-25 1-25 Binder 3 PreGel 5.3 5.30 5.30 Pore-former Graphite 15.3 15.3 15.3 Scavenger Sr(NO.sub.3).sub.2 solution 7.2 1.43-45.5 0.143-251  

(55) TABLE-US-00003 TABLE 3 Commercial Name Chemical wt % Manufacturer AMP95 2-Amino-2-methyl-1-Propanol 95% Angus Chemical Water 5% Company, USA CERDEC Propanol ≧60% Ferro Corporation, 80858 Hydroxy propyl cellulose ether USA CERDEC Ethanol, & 2-(2-ethoxyethoxy)- Ferro Corporation, 80683 ethanol USA Hydroxy propyl cellulose ether PreGel CERDEC 80858 46% Ceramic Fuel Cells CERDEC 80683 46% Ltd, Australia Crayvallac Super 8% Crayvallac Micronised Polyamide wax Arkrma, France Super DGME Diethylene Glycol Monoethyl DOW Chemicals, ether USA LSCo Lanthanum Strontium Cobalt Fuel Cell Oxide Materials, USA Nickel Ni Novamet Specialty Products Corporation, USA

(56) Referring now to the graphs, in FIG. 2 free strontium oxide is provided only in the anode cell contact layer 42 of the cell assembly 10 and provides a substantial reduction in the degradation in electrical output from the assembly over the first 2000 hours of operation relative to no free SrO.

(57) Referring to FIG. 3, the free strontium oxide is provided in the cell assembly 10 only in the cathode cell contact layer 26, and may be seen to reduce the electrical output degradation from about 5% (when no free SrO is present) at nearly 2000 hours to under 2%.

(58) FIG. 4 duplicates the results of FIG. 3, but also shows the effect of additionally including free strontium oxide dispersed in the cathode shield layer 24. With the free strontium oxide dispersed in both the cathode cell contact layer 26 and the cathode shield layer 24, the electrical output degradation is limited to no more than 1% over nearly 2000 hours.

(59) In FIG. 5, the electrical output degradation of the cell assembly 10 with no free strontium oxide present is contrasted over 2500 hours with providing free strontium oxide at positions 4, 5, 6 and 7 on the cathode side only, at positions 1, 2 and 3 on the anode side only, at positions 2, 3, 4, 5 and 6 (so no free strontium oxide interconnect plate coatings on either side), and at all of positions 1 to 7. It may be seen that over this time frame, the cell electrical output degradation decreases from about 6.5% with no strontium oxide present to about 4.8% with free strontium oxide only present to the cathode side, to about 1.3% for each of the tests in which free strontium oxide is only present to the anode side and is dispersed in all four contact layers and in the shield layer, to about 0.1% when free strontium oxide is present in all 7 positions.

(60) FIG. 6 illustrates the results of another test of the cell assembly 10 in which free strontium oxide is provided at all 7 positions. Over about 3700 hours, the cell electrical output degradation remained less than 1%. In contrast, at least by 3000 hours the test showed that with no free strontium oxide present the cell electrical output degradation was greater than 5%.

(61) FIG. 7 illustrates another short term test of the cell assembly 10, over about 800 hours, contrasting the provision of no free strontium oxide in the assembly with individual tests where free strontium oxide is provided at positions 1, 2, 3, 4 and 5, respectively. It may be see that in this test, the provision of free strontium oxide at position 1 only, as a coating on the anode side interconnect plate, improved the electrical output over the initial 450 hours, with negligible degradation in output thereafter.

(62) FIG. 8 is identical to FIG. 7 except that it also shows the results of a test in which free strontium oxide was provided at all of positions 1 to 7. In this test, the final outcome at about 800 hours for providing the free strontium oxide at all 7 positions was a cell output degradation of about 0.8%, worse than when it was provided only at position 1. The reason for this is not clear to the inventors.

(63) FIG. 9 shows the results of a test of a stack with 7 layers of cell assembly 10 with different proportions of free strontium oxide present in 3 layers of the cell 12, the anode contact layer 42 (position 3), the cathode shield layer 24 (position 4) and the cathode contact layer 26 (position 5). The wt % of free strontium oxide in each layer is identified in the Figure by the formula: Layer—#: A/B/C where layer—#=the layer number; A=the SrO wt % in the anode contact layer; B=the SrOwt % in the cathode shield layer; and C=the SrOwt % in the cathode contact layer.

(64) It may be seen from FIG. 9 that all of the layers showed a cell electrical output degradation of less than 1% over the 500 hours tested, but that layers 1, 3, 4 and 7 all showed an improvement in electrical output. Layer 2 showed overall no change in electrical output after 500 hours, while layers 5 and 6 (with no free strontium oxide in the anode contact layer) showed higher cell output degradations at 500 hours of 0.8% and 0.5% respectively. These may be contrasted with the results in FIGS. 7 and 8, where with no free strontium oxide present at all in the cell assembly the cell output degradation over a similar period was about 2% and over 800 hours approached 4%.

(65) FIG. 10 illustrates another short term test of the cell assembly 10, over about 340 hours, contrasting the provision of no free strontium oxide in the assembly with individual tests where free oxide is provided at positions 2, 3, 4, 5 and 6, respectively. In this test, the free oxide is in three different forms, strontium oxide from strontium nitrate precursor, strontium oxide from strontium carbonate precursor and calcium oxide from calcium carbonate precursor. The strontium nitrate is introduced in the manner described above for positions 2 to 6. The strontium carbonate and calcium carbonate are also added in exactly the same way using the compositions set out in Table 4 and 5, respectively, to achieve the same level of active metal content as in the strontium nitrate.

(66) It may be seen from FIG. 10 that in this test the cell electrical output degraded by about 1.6% over the 340 hours of the test when no free oxide is added, while the output from the cells with strontium oxide from strontium nitrate and calcium oxide from calcium carbonate in positions 2 to 6 degraded by about 0.2% over the same period. On the other hand, the provision of strontium oxide from strontium carbonate at positions 2 to 6 improved the cell output over the same period by about 0.5%.

(67) TABLE-US-00004 TABLE 4 FORMULATIONS WITH SrCO.sub.3 Material Current Weight, g Positive side Porous Layer ink Conducting Phase LSCo 31.6 Binder 1 Cerdec 80683 20.2 Binder 2 Cerdec 80858 20.2 Solvent DGME 9.5 Pore-former Graphite 15.2 Scavenger SrCO.sub.3 3.3 Positive side Shield Layer ink Conducting Phase LSCo 49.6 Binder 1 Cerdec 80683 20.8 Binder 2 Cerdec 80858 20.8 Solvent DGME 3.6 Scavenger SrCO.sub.3 5.1 Negative side Porous Layer ink Conducting Phase Ni 38.9 Binder 1 Cerdec 80683 18.2 Binder 2 Cerdec 80858 18.2 Solvent DGME 6.1 Pore-former Graphite 16.6 Scavenger SrCO.sub.3 2.0

(68) TABLE-US-00005 TABLE 5 FORMULATIONS WITH CaCO.sub.3 Material Current Weight, g Positive side Porous Layer ink Conducting Phase LSCo 26.9 Binder 1 Cerdec 80683 29 Binder 2 Cerdec 80858 29 Pore-former Graphite 12.9 Scavenger CaCO.sub.3 2.2 Positive side Shield Layer ink Conducting Phase LSCo 47.2 Binder 1 Cerdec 80683 21.9 Binder 2 Cerdec 80858 21.9 Solvent DGME 5.0 Scavenger CaCO.sub.3 4.0 Negative side Porous Layer ink Conducting Phase Ni 41.9 Binder 1 Cerdec 80683 15.5 Binder 2 Cerdec 80858 15.5 Solvent DGME 6.1 Pore-former Graphite 18.0 Scavenger CaCO.sub.3 3.0

(69) FIG. 11 contrasts the effect on output degradation over 12,000 hours of the provision of strontium oxide derived from strontium nitrate solution in one or more of positions 3, 4 and 5 in a stack comprising 51 cells similar to that of FIG. 1 with output degradation over the same period when no free oxide scavenger material is provided. This test is effectively an extension of the test described with reference to FIG. 9, with the layer materials for the anode contact layer (position 3), the shield layer (position 4) and the cathode contact layer (position 5) being prepared as described with reference to Table 2.

(70) The test showed that over the 12,000 hours the cell with no free oxide present suffered degradation in electrical output of 10.6%. On the other hand, with free strontium oxide at various of positions 3, 4 and 5 the degradation over the same period was 6.2% at position 3 only, 5.1% at position 5 only, 4.7% at positions 4 and 5, and 3.9% at positions 3, 4 and 5. These degradation rates are the average of several cell layers having the same configuration.

(71) This shows that over the test period of 500 days the provision of free oxide on each of the cathode and anode sides of the cell significantly reduced the degradation in electrical output of the cell—by well over 50% with the free oxide at position 5 only, increasing by about another 4% when the free oxide is both positions 4 and 5 on the cathode side, and by about 45% with the free oxide at position 3 on the anode side only. Furthermore, when the free oxide is provided at all 3 of positions 3, 4 and 5 on the anode and cathode sides the degradation was reduced by almost two thirds over the 500 days.

(72) FIG. 12 contrasts the effect on cell output degradation over 2,000 hours of the provision of free oxide derived from different precursors with output degradation over the same period when no free oxide scavenger material is provided.

(73) Four different precursor materials and forms were tested: Sr(NO.sub.3).sub.2 solution; CaCO.sub.3 powder; Sr(NO.sub.3).sub.2 powder; and SrCO.sub.3 powder. The precursors were tested in the same positions in respective cells of a stack, namely positions 3, 4 and 5. The layers comprising the Sr(NO.sub.3).sub.2 solution were prepared as described with reference to Table 2, while each layer comprising one of the powders was prepared according to Table 6, 7 or 8, respectively.

(74) The test showed that over the 2,000 hours the cell with no oxide present suffered degradation in electrical output of 4.3%. On the other hand, the presence of free oxide derived from the different precursors at the three positions 3, 4 and 5 reduced the degradation to the following levels over the same period: 2.7% for Sr (NO.sub.3).sub.2 solution; 2.6% for Sr(NO.sub.3).sub.2 powder; 2.5% for SrCO.sub.3 powder, and 1.7% for CaCO.sub.3 powder. Thus, it may be seen that, for example, the provision of free oxide derived from CaCO.sub.3 as described reduced the cell output degradation by about 60% or 2.5 times over the test period.

(75) TABLE-US-00006 TABLE 6 Formulations with CaCO.sub.3 Powder Material Current Weight, g Cathode Contact Layer Conducting Phase LSCo 32.68 Binder 1 Cerdec 80683 22.87 Binder 2 Cerdec 80858 22.87 Solvent DGME 3.4 Pore-former Graphite 15.87 Scavenger CaCO.sub.3 2.31 Shield Layer Conducting Phase LSCo 51.96 Binder 1 Cerdec 80683 18.6 Binder 2 Cerdec 80858 18.6 Solvent DGME 7.5 Scavenger CaCO.sub.3 3.34 Anode Contact Layer Conducting Phase Ni 35.46 Binder 1 Cerdec 80683 17.41 Binder 2 Cerdec 80858 17.41 Solvent DGME 7.68 Binder 3 PreGel 5.15 Pore-former Graphite 15.23 Scavenger CaCO.sub.3 1.65

(76) TABLE-US-00007 TABLE 7 Formulations with Sr(NO.sub.3).sub.2 Powder Material Current Weight, g Cathode Contact Layer Conducting Phase LSCo 31.89 Binder 1 Cerdec 80683 22.32 Binder 2 Cerdec 80858 22.32 Solvent DGME 3.17 Pore-former Graphite 15.51 Scavenger Sr(NO.sub.3).sub.2 4.78 Shield Layer Conducting Phase LSCo 49.16 Binder 1 Cerdec 80683 17.60 Binder 2 Cerdec 80858 17.60 Solvent DGME 9.00 Scavenger Sr(NO.sub.3).sub.2 6.65 Anode Contact Layer Conducting Phase Ni 35.65 Binder 1 Cerdec 80683 17.50 Binder 2 Cerdec 80858 17.50 Solvent DGME 6.44 Binder 3 PreGel 5.14 Pore-former Graphite 15.34 Scavenger Sr(NO.sub.3).sub.2 2.44

(77) TABLE-US-00008 TABLE 8 Formulations with SrCO.sub.3 Powder Material Current Weight, g Positive side Porous Layer ink Conducting Phase LSCo 32.13 Binder 1 Cerdec 80683 22.49 Binder 2 Cerdec 80858 22.49 Solvent DGME 3.94 Pore-former Graphite 15.60 Scavenger SrCO.sub.3 3.35 Positive side Shield Layer ink Conducting Phase LSCo 51.40 Binder 1 Cerdec 80683 18.40 Binder 2 Cerdec 80858 18.40 Solvent DGME 6.97 Scavenger SrCO.sub.3 4.83 Negative side Porous Layer ink Conducting Phase Ni 36.04 Binder 1 Cerdec 80683 17.70 Binder 2 Cerdec 80858 17.70 Solvent DGME 6.12 Binder 3 PreGel 5.24 Pore-former Graphite 15.49 Scavenger SrCO.sub.3 1.72

(78) Further tests were conducted on the cathode side to assess the ability of free oxides derived from different precursors to absorb chromium emissions from a spinel coated interconnect plate prepared as described with reference to FIG. 1. Chromium is a major poison of SOFC cathode materials, and the ability of the free oxide to scavenge such emissions from the interconnect plate or elsewhere rather than them reacting with the cathode material is an important consideration.

(79) In the first test, the abilities of free oxides derived from different carbonate precursors and an LSCF cathode material to absorb the chromium emissions from the spinel coated interconnect plate were investigated at 800° C. over a period of 50 hours. The alkaline earth metal carbonate salts CaCO.sub.3, BaCO.sub.3 and SrCO.sub.3 were each milled to a particle size typically less than 2 μm and were then turned into inks suitable for screen printing use. The ink formulas were similar to the Positive Side Shield Layer Inks described in Tables 4 and 5, with, carbonate salts fully replacing LSCo. The LSCF cathode material powder was turned into an ink in a similar manner. All inks were screen printed on to a 3 YSZ substrate wafer, forming a coating layer approximately 45 μm thick. The 3YSZ substrate was about 100 μm thick. The LSCF coating was subjected to a firing cycle typical of normal cathode layer fabrication firing as described herein and all the carbonate coatings were dried at 70° C. only to prepare all the coatings for chromium emission testing.

(80) For the chromium emission test, the 3YSZ wafers with various coating materials were broken into small pieces approximately 10 mm×20 mm in size. These small coupons were placed on top of the spinel coated interconnect plate, with the coating materials facing the plate. The interconnect plate, with coated 3YSZ coupons sitting on top, was fired in atmospheric air and allowed to cool. Once cooled, the coating materials were removed from the 3YSZ substrate by dissolving into an acid solution (usually hydrochloric acid), and analysed for chromium content.

(81) The results are given in Table 9 and show that free oxide derived from each of CaCO.sub.3, BaCO.sub.3 and SrCO.sub.3 has a far greater ability to absorb the chromium emissions than the cathode material and therefore that these free oxide materials in or on various layers of the cathode-side chamber of a device such as is shown in FIG. 1 will be effective in scavenging the chromium emissions before they are able to reach the cathode/electrolyte interface.

(82) TABLE-US-00009 TABLE 9 Material |Cr|.ppm CaCO.sub.3 8440 BaCO.sub.3 6577 SrCO.sub.3 3140 LSCF 134

(83) In the second test, the abilities of free oxides derived from two other precursor materials, SrC.sub.2O.sub.4 and NaOH, were tested in comparison with two LSCF cathode materials. One LSCF material was from an earlier purchase batch, applied on a 3YSZ substrate as a coating, designated as LSCF coated 3Y—ZrO.sub.2. The other was from a more recent batch, applied on an anode-supported 8YSZ electrolyte substrate (with a spinel barrier layer as described above) as a coating, and designated as LSCF cathode half cell. Both LSCF coatings were applied by screen printing followed by a sinter firing typical for LSCF cathode fabrication. The first precursor material SrC.sub.2O.sub.4 was provided as an ethanol based SrC.sub.2O.sub.4 slurry impregnated into the porous LSCF layer of the LSCF cathode half cell, and the second precursor material NaOH was provided as a 0.5M NaOH aqueous solution infiltrated into the porous LSCF layer of the LSCF cathode half cell. Both LSCF coatings, as well as the SrC.sub.2O.sub.4 slurry impregnated LSCF cathode half cell and the NaOH solution infiltrated LSCF cathode half cell, were tested to absorb chromium emission from the spinel coated interconnect plate at 650° C. over a period of 20 hours. The chromium emission test set-up was similar to that described for the first test.

(84) The test was run twice and the results are given in Table 10. They show that the free oxide derived from each of SrC.sub.2O.sub.4 and NaOH has a greater ability to absorb the chromium emissions than the cathode material and therefore that these free oxide materials in or on various layers of the cathode-side structure of a fuel cell or electrolyser such as is shown in FIG. 1 will be effective in scavenging the chromium emissions before they are able to reach the cathode/electrolyte interface.

(85) TABLE-US-00010 TABLE 10 Structure |Cr|.ppm-run 1 |Cr|.ppm-run 2 LSCF coated 3YSZ 84 86 LSCF cathode half cell 194 196 Half cell + SrC.sub.2O.sub.4 slurry 584 406 Half cell + NaOH solution 256 206

(86) The test results for the absorption of chromium species by the half cell with the NaOH solution coating are not substantially better than those for the half cell alone, especially in the second run, but this is as a result of using a weak hydroxide solution. It is believed that using a stronger solution will produce significantly better results.

(87) Aspects of the invention described herein and not currently claimed include features of the third and fourth aspects of the invention defined in the following numbered paragraphs:

(88) 1. An electrochemical energy conversion device comprising a stack of solid oxide electrochemical cells alternating with gas separators, wherein each electrochemical cell comprises a layer of solid oxide electrolyte, a negative electrode-side structure on one side of the electrolyte layer and comprising one or more porous layers including a functional layer of negative electrode material having an interface with the one side of the electrolyte layer, and a positive electrode-side structure on the opposite side of the electrolyte layer and comprising one or more porous layers including a layer of positive electrode material having an interface with the opposite side of the electrolyte layer, wherein said electrochemical cell and a first of the gas separators on the negative electrode side of the electrochemical cell at least partly form therebetween a negative electrode-side chamber and said electrochemical cell and a second of the gas separators on the positive electrode side of the electrochemical cell at least partly form therebetween a positive electrode-side chamber, and wherein chemically unbound material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the negative electrode-side structure, the first gas separator and any other structure of the electrochemical energy conversion device forming the negative electrode-side chamber, the unbound material acting to reduce degradation of electrochemical performance on the negative electrode side of the electrochemical energy conversion device during use of the device, and wherein if the chemically unbound material is provided in the functional layer of negative electrode material there is no chemically unbound material present at the interface of that layer with the electrolyte layer.

(89) 2. An electrochemical energy conversion device according to paragraph 1, wherein the unbound material is a scavenger material that is accessible to negative electrode poisons in the atmosphere in the negative electrode-side chamber during use of the device and is more reactive with the poison than is the negative electrode material.

(90) 3. An electrochemical energy conversion device according to paragraph 1 or 2, wherein the unbound material is provided in an unbound material coating on one or more of the negative electrode-side structure, the first gas separator and said any other structure forming the negative electrode-side chamber.

(91) 4. An electrochemical energy conversion device according to paragraph 3, wherein said any other structure forming the negative electrode-side chamber comprises one or more of a separate conductor layer and a separate compliant layer between the first gas separator and the negative electrode-side structure.

(92) 5. An electrochemical energy conversion device according to paragraph 3 or 4, wherein the unbound material coating is discontinuous.

(93) 6. An electrochemical energy conversion device according to any one of paragraphs 3 to 5, wherein the unbound material coating has a thickness of about 0.01 to 250 μm.

(94) 7. An electrochemical energy conversion device according to any one of paragraphs 3 to 5, wherein the unbound material coating has a thickness of about 0.01 to 50 μm.

(95) 8. An electrochemical energy conversion device according to any one of paragraphs 1 to 7, wherein the unbound material is dispersed in at least one of the one or more porous layers of the negative electrode-side structure.

(96) 9. An electrochemical energy conversion device according to any one of paragraphs 1 to 8, wherein the one or more porous layers of the negative electrode-side structure comprises, in addition to the functional layer of negative electrode material, a negative electrode-side layer of electrical contact material.

(97) 10. An electrochemical energy conversion device according to paragraph 9, wherein the unbound material is provided in the layer of electrical contact material.

(98) 11. An electrochemical energy conversion device according to any one of paragraphs 1 to 10, wherein the one or more porous layers of the negative electrode-side structure comprises, in addition to the functional layer of negative electrode material, a negative electrode-side layer of substrate material.

(99) 12. An electrochemical energy conversion device according to paragraph 11, wherein the unbound material is provided in the layer of substrate material.

(100) 13. An electrochemical energy conversion device according to any one of paragraphs 1 to 12, wherein the first gas separator comprises a dense substrate and one or more porous layers on a side of the substrate facing the negative electrode-side chamber, and wherein the unbound material is provided in at least one of the one or more porous layers of the first gas separator.

(101) 14. A electrochemical energy conversion device according to any one of paragraphs 1 to 13, wherein the first gas separator comprises a dense substrate and a protective coating on a side of the substrate facing the negative electrode-side chamber and in contact with the substrate, and wherein the unbound material is provided in the protective coating.

(102) 15. An electrochemical energy conversion device according to paragraph 13 or 14, wherein the unbound material is dispersed in one or both of said at least one of the one or more porous layers of the first gas separator and the protective coating of the first gas separator.

(103) 16. An electrochemical energy conversion device according to any one of paragraphs 8 to 15, wherein the unbound material in any one porous layer or in the protective coating is provided at a level in the range of about 0.1 to 65 vol % of the total solid content of the layer or coating.

(104) 17. An electrochemical energy conversion device according to paragraph 16, wherein the range is about 1 to 25 vol %.

(105) 18. An electrochemical energy conversion device according to any one of paragraphs 1 to 17, wherein the unbound material comprises free oxide selected from one or more of SrO, CaO, BaO, MgO, Na.sub.2O and K.sub.2O.

(106) 19. An electrochemical energy conversion cell comprising a layer of solid oxide electrolyte, a negative electrode-side structure on one side of the electrolyte layer and comprising one or more porous layers including a functional layer of negative electrode material having an interface with the one side of the electrolyte layer, and a positive electrode-side structure on the opposite side of the electrolyte layer and comprising one or more porous layers including a layer of positive electrode material having an interface with the opposite side of the electrolyte layer, wherein chemical unbound material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on the negative electrode-side structure and acts to reduce degradation of electrochemical performance on the negative electrode side of the electrochemical energy conversion cell during use of the cell, and wherein if the chemically unbound material is provided in the functional layer of negative electrode material there is no chemically unbound material present at the interface of that layer with the electrolyte layer.

(107) 20. A electrochemical energy conversion cell according to paragraph 1, wherein the unbound material is a scavenger material that is accessible to negative electrode poisons in atmosphere contacting the negative electrode-side structure during use of the cell and is more reactive with the poisons than is the negative electrode material.

(108) 21. An electrochemical energy conversion cell according to paragraph 19 or 20, wherein the unbound material is provided in a discontinuous unbound material coating on the negative electrode-side structure.

(109) 22. An electrochemical energy conversion cell according to paragraph 21, wherein the unbound material coating has a thickness of about 0.01 to 250 μm.

(110) 23. An electrochemical energy conversion cell according to paragraph 21, wherein the unbound material coating has a thickness of about 0.01 to 50 μm.

(111) 24. An electrochemical energy conversion cell according to any one of paragraphs 19 to 23, wherein the unbound material is dispersed in at least one of the one or more porous layers of the negative electrode-side structure.

(112) 25. An electrochemical energy conversion ceil according to any one of paragraphs 19 to 24, wherein the one or more porous layers of the negative electrode-side structure comprises, in addition to the functional layer of negative electrode material, a negative electrode-side layer of electrical contact material.

(113) 26. An electrochemical energy conversion cell according to paragraph 25, wherein the unbound material is provided in the layer of electrical contact material.

(114) 27. An electrochemical energy conversion cell according to any one of paragraphs 19 to 26, wherein the one or more porous layers of the negative electrode-side structure comprises, in addition to the functional layer of negative electrode material, a negative electrode-side layer of substrate material.

(115) 28. An electrochemical energy conversion cell according to paragraph 27, wherein the unbound material is provided in the layer of substrate material.

(116) 29. An electrochemical energy conversion cell according to any one of paragraphs 19 to 28, wherein the unbound material in any one porous layer is provided at a level in the range of about 0.1 to 65 vol % of the total solid content of the layer.

(117) 30. An electrochemical energy conversion cell according to paragraph 29, wherein the range is about 1 to 25 vol %.

(118) 31. An electrochemical energy conversion cell according to any one of paragraphs 19 to 30, wherein the unbound material comprises free oxide selected from one or more of SrO, CaO, BaO, MgO, Na.sub.2O and K.sub.2O.

(119) Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, it is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

(120) Whilst the present invention has been described with reference to specific embodiments and planar fuel cells, it will be appreciated that such embodiments are merely exemplary, and other embodiments other than those described herein will be encompassed by the invention as defined by the appended claims.

(121) The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

(122) Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.