Method for deposition of ceramic films

Abstract

The present invention is concerned with methods for the deposition of ceramic films on ceramic or metallic surfaces, particularly the deposition of sub-micron thickness ceramic films such as films of stabilised zirconia and doped ceria such as CGO (cerium gadolinium oxide). The present invention is particularly useful in the manufacture of high and intermediate temperature operating fuel cells including solid oxide fuel cells (SOFC) and also metal supported intermediate temperature SOFC operating in the 450-650 C. range.

Claims

1. A method for depositing at least two layers of an SOFC electrolyte on a substrate that is a main electrolyte layer of the electrolyte, in which the at least two layers are metal oxide crystalline ceramic layers deposited upon the substrate, said substrate being a substantially impermeable mixed ionic electronic conducting electrolyte ceramic material, the method comprising the steps of: (i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto a surface of the substrate to define a layer of said solution of said soluble salt precursor on said surface; (ii) drying said solution of said soluble salt precursor to define a layer of said soluble salt precursor on said surface; (iii) heating said soluble salt precursor on said surface to a temperature of between 150 and 600 C. to decompose it and form a layer of metal oxide film on said surface; (iv) sequentially depositing, drying and heating at between 100 and 600 C. at least one additional layer of the solution of the soluble salt precursor to form a plurality of layers of metal oxide film; and (v) firing said substrate with said plurality of layers of metal oxide film at a temperature of 500-1100 C. to crystallise said plurality of layers of metal oxide film into a layer of metal oxide crystalline ceramic bonded to said substrate, said layer of metal oxide crystalline ceramic being ion-permeable, electrically insulating, and gas-permeable and said layer of metal oxide crystalline ceramic providing at least a part of an interlayer for the SOFC electrolyte; (vi) sequentially depositing, drying and heating at between 100 and 600 C. at least one further solution of a further soluble salt precursor of a further metal oxide crystalline ceramic to form a plurality of layers of a further metal oxide film on the interlayer; (vii) firing said substrate with said plurality of layers of a further metal oxide film at a temperature of 500-1100 C. to crystallise said plurality of layers of a further metal oxide film into a further layer of metal oxide crystalline ceramic, said further layer of metal oxide crystalline ceramic being a mixed ionic electronic conducting electrolyte material and said further layer of metal oxide crystalline ceramic providing at least a part of a barrier layer between the interlayer and a subsequently deposited cathode layer; and (viii) subsequently depositing the cathode layer; wherein: each of steps (ii), (iii), (v) and (vii) is performed in an air atmosphere; said interlayer of metal oxide crystalline ceramic is thinner than the main electrolyte layer provided by the substantially impermeable mixed ionic electronic conducting electrolyte ceramic material substrate; and said interlayer is also to electrically insulate the subsequently deposited cathode layer from the main electrolyte layer.

2. A method according to claim 1, wherein said main electrolyte layer is a CGO electrolyte layer.

3. A method according to claim 1, wherein: in steps (i)-(v) said soluble salt precursor is selected from at least one of the group consisting of: zirconium acetylacetonate, scandium nitrate, and yttrium nitrate; and in steps (vi)-(vii) said further soluble salt precursor is selected from at least one of the group consisting of: cerium acetylacetonate and gadolinium nitrate.

4. A method according to claim 1, wherein each deposition of a solution of a soluble salt precursor of a metal oxide crystalline ceramic is undertaken with its receiving surface having a temperature of 10-100 C.

5. A method according to claim 1, wherein said surface of said substrate is substantially impermeable to the soluble salt precursor deposited thereon.

6. A method according to claim 1, wherein at least one of the layers of metal oxide crystalline ceramic or further metal oxide crystalline ceramic comprises doped stabilised zirconia.

7. A method according to claim 6, wherein that at least one layer of metal oxide crystalline ceramic or further metal oxide crystalline ceramic is a ceramic selected from the group consisting of: scandia stabilised zirconia (ScSZ), yttria stabilised zirconia (YSZ), scandia ceria co-stabilised zirconia (ScCeSZ), and scandia yttria co-stabilised zirconia (ScYSZ).

8. A method according to claim 1, wherein at least one of the layers of metal oxide crystalline ceramic or further metal oxide crystalline ceramic comprises rare earth oxide doped ceria.

9. A method according to claim 8, wherein at least one of the layers of metal oxide crystalline ceramic or further metal oxide crystalline ceramic is a mixed ionic electronic conducting electrolyte material metal oxide crystalline ceramic is selected from the group consisting of: samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), praseodymium doped ceria (PDC), and samaria-gadolinia doped ceria (SGDC).

10. A method according to claim 1, wherein the soluble salt precursor of steps (i)-(iv) is selected from at least one of the group consisting of: zirconium acetylacetonate, scandium nitrate, yttrium nitrate, cerium acetylacetonate and gadolinium nitrate.

11. A method according to claim 1, wherein a solvent for the soluble salt precursor or the further soluble salt precursor is selected from at least one of the group consisting of: methanol, ethanol, propanol, methoxypropanol, acetone and butyl carbitol.

12. A method according to claim 1, additionally comprising prior to step (ii) the step of allowing said solution deposited onto said surface to stand for a period of between 5 and 60 seconds.

13. A method according to claim 1, wherein said layer of said soluble salt precursor on said surface has a thickness between 10 and 999 nm.

14. A method according to claim 1, wherein said plurality of layers of metal oxide film each have a thickness less than 1 micron.

15. A method according to claim 1, wherein said layer of metal oxide crystalline ceramic bonded to said substrate has a thickness between 100 and 999 nm.

16. A method according to claim 1, wherein said layer of metal oxide crystalline ceramic bonded to said substrate is at least 90% dense.

17. A method according to claim 1 wherein at least 90% of a solvent in the deposited solution of said soluble salt precursor and a solvent in the deposited further solution of said further soluble salt precursor is removed by the end of its respective drying step, prior to its respective heating step.

18. A method according to claim 1, wherein the deposited solution of the soluble salt precursor of the metal oxide crystalline ceramic and the deposited further solution of the further soluble salt processor of the further metal oxide crystalline ceramic are deposited by a method selected from the group consisting of: spraying, dipping, inkjet printing and spin-coating.

19. A method according to claim 1, wherein said substrate is CGO, said interlayer is ScYSZ or ScSZ, and said barrier layer is CGO.

20. A surface of a substrate, wherein said substrate is a substantially impermeable mixed ionic electronic conducting electrolyte material, and said surface is a ceramic surface, said surface having deposited upon it at least two layers of metal oxide crystalline ceramic by way of the method according to claim 1.

21. A method for depositing at least two layers of an SOFC electrolyte on a substrate that is a main electrolyte layer of the electrolyte, in which the at least two layers are metal oxide crystalline ceramic layers deposited upon the substrate, said substrate being a substantially impermeable mixed ionic electronic conducting electrolyte ceramic material, the method comprising the steps of: (i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto a surface of the substrate to define a layer of said solution of said soluble salt precursor on said surface; (ii) drying said solution of said soluble salt precursor to define a layer of said soluble salt precursor on said surface; (iii) heating said soluble salt precursor on said surface to a temperature of between 150 and 600 C. to decompose it and form a layer of metal oxide film on said surface; (iv) sequentially depositing, drying and heating at between 100 and 600 C. at least one additional layer of the solution of soluble salt precursor to form a plurality of layers of metal oxide film; (v) sequentially depositing, drying and heating at between 100 and 600 C. at least one further solution of a further soluble salt precursor of a further metal oxide crystalline ceramic to form a plurality of layers of a further metal oxide film; and (vi) firing said substrate with said plurality of layers of metal oxide film and said plurality of layers of said further metal oxide film at a temperature of 500-1100 C. to crystallise a) said plurality of layers of metal oxide film into a layer of metal oxide crystalline ceramic bonded to said substrate, said layer of metal oxide crystalline ceramic being ion-permeable, electrically insulating, and gas-impermeable and said layer of metal oxide crystalline ceramic providing at least a part of an interlayer for the SOFC electrolyte, and b) said plurality of layers of said further metal oxide film into a further layer of metal oxide crystalline ceramic, said layer of metal oxide crystalline ceramic being a mixed ionic electronic conducting electrolyte material and said layer of metal oxide crystalline ceramic providing at least a part of a barrier layer between the interlayer and a subsequently deposited cathode layer; and (vii) subsequently depositing the cathode layer; wherein; each of steps (ii), (iii), and (vi) is performed in an air atmosphere; said interlayer of metal oxide crystalline ceramic is thinner than the main electrolyte layer provided by the substantially impermeable mixed ionic electronic conducting electrolyte ceramic material substrate; and said interlayer is also to electrically insulate the subsequently deposited cathode layer from the main electrolyte layer.

22. A method according to claim 21, wherein said main electrolyte layer is a CGO electrolyte layer.

23. A method according to claim 21, wherein: in step (i)-(iv) said soluble salt precursor is selected from at least one of the group consisting of: zirconium acetylacetonate, scandium nitrate, and yttrium nitrate; and in step (v) said further soluble salt precursor is selected from at least one of the group consisting of: cerium acetylacetonate and gadolinium nitrate.

Description

(1) FIG. 1 shows a scanning electron microscopy (SEM) cross-section at 25000 magnification of part of an SOFC electrolyte manufactured by the method of the present invention;

(2) FIG. 2 shows an SEM cross-section at 850 magnification of part of an SOFC electrolyte manufactured by the method of the present invention near the edge of the anode, and showing overlap of the electrolyte onto the steel substrate;

(3) FIG. 3 shows a comparison of performance of metal supported SOFC cells with and without ScSZ barrier layers operating at 570 C. at high fuel utilisation. X-axis shows Current density/mA/cm.sup.2; left-most Y-axis shows Cell voltage/V; right-most Y-axis shows Power density/mW/cm.sup.2. Reference signs: 100Power density with barrier; 110Power density without barrier; 120Cell voltage with barrier; 130Cell voltage without barrier;

(4) FIGS. 4-7 show various fuel cell layer structures that can be achieved with the method of the present invention;

(5) FIG. 8 shows a top view of a graded thickness ScYSZ layer;

(6) FIG. 9 shows a cross-section through I-I of FIG. 8;

(7) FIG. 10 shows a fuel cell layer structure having a graded thickness ScYSZ layer;

(8) FIG. 11 shows a plot of in-use electrolyte temperature of fuel cell layer structure of FIG. 10 between points A and B. X-axis shows temperature, left side corresponds to point A, right side corresponds to point B; Y axis shows electrolyte temperature;

(9) FIG. 12 shows a top view of a graded thickness ScYSZ layer; and

(10) FIG. 13 shows a cross-section through II-II of FIG. 12.

(11) In a first embodiment, a ferritic stainless steel foil substrate (as shown in e.g. FIGS. 4-7) defining a perforated region surrounded by a non-perforated region is provided upon which has been deposited an anode layer and a gas impermeable, dense, CGO electrolyte layer 10 which is 10-15 micron thick on top of the anode layer, as taught in GB2434691 (foil substrate 4, anode layer 1a and electrolyte layer 1e) and WO 02/35628. In other embodiments (not shown) perforated foil substrates upon which is deposited an anode layer and a gas impermeable, dense electrolyte layer are used (GB2440038, GB2386126, GB2368450, U.S. Pat. No. 7,261,969, EP1353394, and U.S. Pat. No. 7,045,243). In a further embodiment (not shown) the graded metal substrate of US20070269701 is used.

(12) An interlayer 20 of crystalline ceramic scandia yttria co-stabilised zirconia (10Sc1YSZ; (Sc.sub.2O.sub.3).sub.0.1(Y.sub.2O.sub.3).sub.0.01(ZrO.sub.2).sub.0.89) is then formed on top of the CGO layer by performing steps (a)-(f) below. This is a particularly useful interlayer material since zirconium salts are not highly soluble and have a tendency to precipitate out. Thus, the dopant levels in the deposited layer of soluble salt precursor can exhibit a slight variance and the addition of 1% Yttria helps avoid the phase instability which can occur in >9% ScSZ.

(13) The steps are: (a) air atomised spraying a layer of 0.1M cation concentration solution of Sc(NO.sub.3).sub.3 and Y(NO.sub.3).sub.3 and Zr(C.sub.5H.sub.7O.sub.2) in 90% volume ethanol and 10% volume methoxypropanol (soluble salt precursors which will form the scandia yttria co-stabilised zirconia) at RTP onto the CGO layer. (b) drying the soluble salt precursor layer at RTP in air for 60 seconds during which period the soluble salt precursor evens out across the surface, followed by further drying at 100 C. for 30 seconds. (c) heating the soluble salt precursor to 550 C. over a total period of 60 seconds using an infra-red (IR) heating lamp which decomposes and semi-crystalises it to form a layer about 125 nm thick of a semi-crystalline scandia yttria co-stabilised zirconia film. (d) repeating steps (a)-(c) 4 times, the substrate and metal oxide film being cooled to a temperature of 35-80 C. prior to each repeat of step (a), to give a metal oxide and semi-crystalline film having a total thickness of about 500 nm. This film does not have any cracks in it and is suitable for further processing. (e) firing at 800 C. for 60 minutes in air, the metal oxide film of scandia yttria co-stabilised zirconia forms a fully crystalline ceramic layer 20 of scandia yttria co-stabilised zirconia, having a thickness of about 400 nm. (f) Repeating steps (a)-(e) once more to achieve a final layer 20 thickness of about 800 nm

(14) The next steps are: (g) the repeating of steps (a)-(e) once more but this time depositing a layer 30 of CGO on top of the previously deposited crystalline ceramic layer of scandia yttria co-stabilised zirconia. Specific conditions are: 0.1M cation concentration Ce(C.sub.5H.sub.7O.sub.2) and gadolinium nitrate in 70% volume ethanol and 30% volume methoxypropanol and spraying, depositing and processing as before but using a final crystallisation firing temperature of 980 C. to achieve a CGO layer with a final thickness of 250 nm. This layer acts as a barrier layer between the scandia yttria co-stabilised zirconia layer and a subsequently deposited cathode layer. (h) finally, a cathode layer 40 is then deposited on top of the previously deposited CGO layer. This is done by screen-printing an LSCF cathode and processing it in accordance with WO2006/079800. This layer has a thickness of about 50 m.

(15) Thus, an overall structure is created comprising the following layers deposited on the ferritic stainless steel support: (A) NiO-CGO anode; (B) CGO (10) (C) scandia yttria co-stabilised zirconia (20) (D) CGO (30) (E) cathode (40)

(16) The results of the method detailed above are shown in FIG. 1. The main CGO layer 10 has been deposited on top of the anode layer (not shown in FIG. 1).

(17) It should be noted that the maximum processing temperature used in the manufacture of the zirconia layer and the CGO layer was 980 C., which is significantly below the temperature at which zirconia and CGO start to react, which is 1200 C.

(18) The results of the above method are shown in the Figures. On top of main CGO layer 10 is ScYSZ interlayer 20 which has a thickness of 800 nm. Testing shows it to have high ionic conductivity and good phase stability. ScYSZ interlayer 20 has density comparable with main CGO layer 10 and a very good interface with main CGO layer 10. This layer is present to electronically insulate cathode 40 from the main electrolyte layer 10 whilst allowing the passage of an ionic current, and thus eliminate the internal short-circuiting inherent in operating a CGO or other doped ceria electrolyte alone.

(19) On top of interlayer 20 is a CGO interlayer 30 which has a thickness of 0.25 m. This layer is present to separate the cathode material from the zirconia of interlayer 20, improving the catalytic activity of cathode 40 by an effective electrolyte-cathode interface of high ionic conductivity. This also avoids potentially detrimental chemical interactions between cathode 40 and the zirconia of interlayer 20 either during processing or possibly during service.

(20) An important aspect of this embodiment is the high density of interlayer 20. This high density is required for the effective blocking of internal short circuiting, because it has been shown that CGO, particularly in its reduced state, has significant catalytic activity for the reduction of oxygen. Thus, if the interlayer 20 is not gas-impermeable (say, is <=93% dense), oxygen will diffuse through interlayer 20 and reduce on the top of main CGO layer 10, allowing a path for internal short-circuiting, bypassing cathode 40 entirely.

(21) As can be seen from FIG. 2, it shows a cross-section of the edge (60) of an anode in a fuel cell. On the right hand side of the figure there is a clear anode layer deposited upon ferritic stainless steel substrate 50, but towards the left hand side of the figure main CGO layer 10 is deposited directly onto ferritic stainless steel substrate 50.

(22) FIG. 2 also shows the relative scale of main CGO layer as compared to ScYSZ interlayer 20 and CGO interlayer 30. Cathode 40 can be seen to comprise cathode active layer 70 and cathode current collector 80.

(23) FIG. 3 shows a comparison of VI and power curves of first and second metal supported SOFCs with CGO main electrolytes, showing the benefit of a thin ScYSZ barrier layer deposited using the method of the present invention.

(24) The first SOFC is constructed according to the first embodiment (above). The second SOFC is constructed identically to the first SOFC except that it does not include the scandia yttria co-stabilised zirconia layer 20 and the additional CGO layer 30 on top.

(25) As can be seen from FIG. 3, cell voltages and power densities are significantly increased. In particular, there is a much higher open-circuit voltage as a result of the blockage of electronic leakage through the CGO electrolyte.

(26) In a second embodiment (not shown), the first CGO layer is a 2% Co-doped CGO layer. The doping is used as a sintering aid to aid densification of the CGO electrolyte at a lowered sintering temperature compared to an un-doped CGO electrolyte. The ScYSZ interlayer 20 and the additional CGO layer 30 are prepared and deposited and processed in the same way as for the first embodiment.

(27) In a third embodiment the deposition of the layer of CGO on top of the previously deposited layer of scandia yttria co-stabilised zirconia is preceded by a repeat of steps (a)-(e) in which an additional layer of scandia yttria co-stabilised zirconia is deposited (i.e. on top of the previously deposited layer of scandia yttria co-stabilised zirconia).

(28) In a fourth embodiment, a graded thickness layer of metal oxide crystalline ceramic is provided on a CGO surface for subsequent use in the manufacture of an IT-SOFC. In-use, the layer of metal oxide crystalline ceramic is an electrolyte interlayer, the interlayer thickness being graded across the in-use gas flow path of the fuel cell in order to improve the low temperature cell performance without allowing excessive electronic leakage.

(29) FIGS. 4-7 show various fuel cell layer structures that can be achieved with the method of the present invention.

(30) As can be seen from FIG. 4, in a fourth embodiment a fuel cell is to be constructed, and a ferritic stainless steel metal substrate 200 with an anode layer 210 on top of it and an electrolyte layer 220 are provided. Electrolyte layer 220 surrounds anode layer 210 in order to prevent gas flowing through anode 210 between fuel side 230 and oxidant side 240. Scandia yttria co-stabilised zirconia crystalline ceramic layer 250 is then deposited on top of ceramic CGO layer 220, and CGO crystalline ceramic layer 260 is then deposited on top of layer 250. The spraying steps used in the deposition of layers 250 and 260 results in a layer cake type structure.

(31) Subsequent to the deposition of layers 250 and 260, the fuel cell is completed with the addition of cathode assembly 270.

(32) FIG. 5 shows a fifth embodiment in which CGO layer 260 overlaps and contains the other layers 220, 250. Notably, in this embodiment the layer of metal oxide crystalline ceramic (CGO layer 260) is deposited both onto the ceramic layer underneath (scandia yttria co-stabilised zirconia layer 250) and also ferritic stainless steel substrate 200. The spraying steps used in this embodiment results in a contained layer type structure.

(33) FIG. 6 shows a sixth embodiment in which scandia yttria co-stabilised zirconia layer 250 overlaps CGO layer 220. Again, in this embodiment the layer of metal oxide crystalline ceramic (in this case scandia yttria co-stabilised zirconia layer 250) is deposited both onto the ceramic layer underneath (CGO layer 220) and also ferritic stainless steel substrate 200. The spraying steps used in this embodiment results in an overlap type structure.

(34) FIG. 7 shows a seventh embodiment in which scandia yttria co-stabilised zirconia layer 250 overlaps CGO layer 220 and in turn is overlapped by CGO layer 260. In this embodiment both layers of metal oxide crystalline ceramic (CGO layer 260 and scandia yttria co-stabilised zirconia layer 250) are deposited onto both the ceramic layer underneath (and scandia yttria co-stabilised zirconia layer 250 and CGO layer 220 respectively) and also ferritic stainless steel substrate 200. The spraying steps used in this embodiment results in a contained overlap type structure.

(35) FIGS. 8 and 9 show an eighth embodiment in which a scandia yttria co-stabilised zirconia (ScYSZ) layer 300 has a graded thickness across the surface of CGO layer 220. As detailed above for the first embodiment, a ferritic stainless steel foil substrate 200 defining a perforated region 201 surrounded by a non-perforated region 202 is provided upon which has been deposited an anode layer 210 and a gas impermeable, dense, CGO electrolyte layer 220 10-15 micron thick on top of the anode layer 210.

(36) The ScYSZ layer 300 is then deposited on top with a first step being a two-stage spraying operation using the basic method as described above for the first embodiment. The layer 300 is crystalline ceramic scandia yttria co-stabilised zirconia (10Sc1YSZ; (Sc.sub.2O.sub.3).sub.0.1(Y.sub.2O.sub.3).sub.0.01(ZrO.sub.2).sub.0.89) and is formed as follows: First, steps (a)-(d) (above) are performed to give a first area 320 of metal oxide film. A mask is then placed over the metal oxide film so as to leave only a portion of it exposed, and steps (a)-(d) are then repeated to define a second area 330 in which the layer of metal oxide film consists two films of metal oxide.

(37) Step (e) is then performed to form crystalline ceramic ScYSZ layer 300. The area 320 has a thickness of about 400 nm and the area 330 has a thickness of about 800 nm.

(38) It should be noted that the crystalline ceramic ScYSZ layer 300 does not totally overlap CGO electrolyte layer 220as can be seen at 310 there is an area where CGO electrolyte layer 220 overlaps anode layer 210 but which is then not overlapped by crystalline ceramic ScYSZ layer 300.

(39) FIG. 10 shows a fuel cell electrolyte structure 340. As detailed above for the first embodiment, a ferritic stainless steel foil substrate 200 defining a perforated region 201 surrounded by a non-perforated region 202 is provided upon which has been deposited an anode layer 210 and a gas impermeable, dense, CGO electrolyte layer 220 10-15 micron thick on top of the anode layer 210.

(40) An ScYSZ layer 350 is then deposited on top in a multi-stage spraying operation using the basic method as described above for the first embodiment. The layer 350 is crystalline ceramic scandia yttria co-stabilised zirconia (10Sc1YSZ; (Sc.sub.2O.sub.3).sub.0.1(Y.sub.2O.sub.3).sub.0.01(ZrO.sub.2).sub.0.89) and is formed by performing steps (a)-(d) above, but with step (a) being accomplished using inkjet printing. This results in a first area 360 of soluble salt precursor which is heated to give a metal oxide film.

(41) A series of repeats of steps (a)-(d) are then performed, each repeat occurring over only a portion of the area from the previous inkjet printing step, thus defining second, third, fourth and fifth areas 370, 380, 390 and 400.

(42) Step (e) is then performed to define a metal oxide crystalline ceramic layer, areas 360, 370, 380, 390 and 400 having thicknesses of about 150, 300, 450, 600 and 750 nm respectively.

(43) Step (g) is then performed to deposit CGO interlayer 410 on top of ScYSZ layer 350 having a thickness of 0.25 m.

(44) Step (h) is then performed to deposit cathode layer 420.

(45) In-use operating temperatures of the CGO layer (electrolyte) 220 are shown in FIG. 11. As can be seen, the gradation in operational temperature is matched by the gradation in thickness of ScYSZ layer 350.

(46) FIGS. 12 and 13 show an embodiment similar to that for FIGS. 8 and 9 in which a crystalline scandia yttria co-stabilised zirconia (ScYSZ) layer 500 has a graded thickness across the surface of CGO layer 220 and where the crystalline ceramic ScYSZ layer 500 does not totally overlap CGO electrolyte layer 220 or the anode layer 210as can be seen at 510 there is an area where CGO electrolyte layer 220 overlaps anode layer 210 but which is then not overlapped by crystalline ceramic ScYSZ layer 500, and at 540 there is an area where CGO electrolyte layer 220 overlaps anode layer 210 but which neither the CGO electrolyte layer 220 or the anode layer 210 is then overlapped by crystalline ceramic ScYSZ layer 500.

(47) As detailed above for the first embodiment, a ferritic stainless steel foil substrate 200 defining a perforated region 201 surrounded by a non-perforated region 202 is provided upon which has been deposited an anode layer 210 and a gas impermeable, dense, CGO electrolyte layer 220 10-15 micron thick on top of the anode layer 210.

(48) The ScYSZ layer 500 is then deposited on top with a first step being a two-stage spraying operation using the basic method as described above for the first embodiment. The layer 500 is crystalline ceramic scandia yttria co-stabilised zirconia (10Sc1YSZ; (Sc.sub.2O.sub.3).sub.0.1(Y.sub.2O.sub.3).sub.0.01(ZrO.sub.2).sub.0.89) and is formed as follows: First, steps (a)-(d) (above) are performed to give a first area 520 of metal oxide film. A mask is then placed over the metal oxide film so as to leave only a portion of it exposed, and steps (a)-(d) are then repeated to define a second area 530 in which the layer of metal oxide film consists two films of metal oxide.

(49) Step (e) is then performed to form crystalline ceramic ScYSZ layer 500. The area 520 has a thickness of about 400 nm and the area 530 has a thickness of about 800 nm.

(50) It should also be noted that the crystalline ceramic ScYSZ layer 500 does not totally overlap CGO electrolyte layer 220 or the anode layer 210as can be seen at 540 there is an area where CGO electrolyte layer 220 overlaps anode layer 210 but which neither the CGO electrolyte layer 220 or the anode layer 210 is then overlapped by crystalline ceramic ScYSZ layer 500.

(51) It will be appreciated that the present invention is not limited to the above examples only, other embodiments falling within the scope of the appended claims being readily apparent to a person of ordinary skill in the art.

REFERENCE SIGNS

(52) 10main CGO layer 20ScSZ interlayer 30CGO interlayer 40cathode 50ferritic stainless steel substrate 60anode edge 70cathode active layer 80cathode current collector 100Power density with barrier 110Power density without barrier 120Cell voltage with barrier 130Cell voltage without barrier 200ferritic stainless steel substrate 201perforated region 202non-perforated region 210anode layer 220CGO layer 230fuel side 240oxidant side 250scandia yttria co-stabilised zirconia layer 260CGO layer 270cathode assembly 300scandia yttria co-stabilised zirconia (ScYSZ) layer 310area of CGO electrolyte layer 220 not overlapped by ceramic ScYSZ layer 300 320first area of scandia yttria co-stabilised zirconia (ScYSZ) layer 300 330second area of scandia yttria co-stabilised zirconia (ScYSZ) layer 300 340fuel cell electrolyte structure 350ScYSZ layer 360first area 370second area 380third area 390fourth area 400fifth area 410CGO interlayer 420cathode layer 500crystalline scandia yttria co-stabilised zirconia (ScYSZ) layer 510area of CGO electrolyte layer 220 not overlapped by ceramic ScYSZ layer 500 520first area of scandia yttria co-stabilised zirconia (ScYSZ) layer 500 530second area of scandia yttria co-stabilised zirconia (ScYSZ) layer 500 540area of anode layer 210 and CGO electrolyte layer 220 not overlapped by ceramic ScYSZ layer 500