GB2490869A

GB2490869A - Cathode ink

Info

Publication number: GB2490869A
Application number: GB1107672.6A
Authority: GB
Inventors: Katie Sarah Howe; Ewan Robin Clark
Original assignee: University of Birmingham
Current assignee: University of Birmingham
Priority date: 2011-05-09
Filing date: 2011-05-09
Publication date: 2012-11-21
Also published as: GB201107672D0

Abstract

A cathode ink comprises ceramic powder particles dispersed in water, and a neutral dispersant. Preferably, the ceramic powder particles comprise lanthanum strontium manganate and yttria-stabilised zirconia. Preferably the lanthanum strontium manganate has particles of size in the range 0.1Âµm-10Âµm and preferably the yttria-stabilised zirconia has particles in the range 0.1Âµm-30Âµm. Preferably the dispersant comprises poly(N-vinylpyrrolidone) (PVP). The ink may further include a binder in particular polyvinyl alcohol and/or polyvinyl butyral. Described is a cathode layer produced using the cathode ink and a solid oxide fuel cell incorporating the cathode layer. A process for forming the cathode ink preferably comprises the steps of mixing the ceramic powder particles and dispersant with water and milling the ceramic powder particles, dispersant and water to stabilise the ink. A process for electrode formation, preferably comprises depositing the cathode ink onto an electrolyte surface and sintering the ceramic particles to form one or more cathode layers. Solid oxide fuel cells may also be formed by providing an anode and electrolyte, forming the electrode as discussed above and forming current connections.

Description

Field

[0001] The invention relates to an ink, in particular to a cathode ink. The ink may form a cathode layer, the cathode layer being generally suitable for electrode formation, for instance in a solid oxide fuel cell. The invention also relates to processes for the formation of these products.

Background

[0002] Solid oxide fuel cells have the potential to provide an environmentally friendly energy source, with high energy conversion efficiency, low emissions and flexibility of fuels.

[0003] To prepare solid oxide fuel cells, it is necessary to develop new cathodic and anodic materials. One solution has been to prepare ink materials which can be deposited onto the electrolyte and dried to form electrode layers. This invention concentrates, in particular, on a new cathode ink material.

[0004] Cathode inks typically contain ceramic powder particles, dispersants, binders and a solvent. Most cathode inks use organic solvents such as acetone, iso-propanol, methyl ethyl ketone and ethanol. Yashiro et al (Application of a thin intermediate cathode layer prepared by inkjet printing for SOFCs, Journal of the European Ceramic Society, 30, (2010), 2093- 2098) describes a water based cathode ink, including an acid dispersant. The acid dispersant is present to stabilise the binders in the cathode ink composition, reducing unwanted side reactions which would otherwise reduce cell efficiency.

[0005] Inks including organic solvents can be difficult to handle due to their toxicity and volatility. In particular, organic solvent based inks often dry too quickly to produce high quality cathode layers. As such, it would be desirable to move away from the use of organic solvents in these inks. In addition, the use of highly acidic or alkaline materials in cathode inks is undesirable as, unless a buffer is used, these components can decompose the other constituents of the inks. The inclusion of a buffer is preferably avoided as this increases the complexity of the ink formulation.

100061 The invention is intended to overcome or ameliorate at least some aspects of these problems.

Summary

100071 Accordingly, in a first aspect of the invention there is provided a cathode ink comprising ceramic powder particles dispersed in water, and a neutral dispersant. It has been found that ceramic powder particles can be dispersed water in the presence of a neutral dispersant to form a stable ink composition.

[0008] It will often be the case that the components of the ink of the invention be readily available to the skilled user, easy to handle, and of minimal toxicity. This will provide an ink which is easy to prepare, easy to handle and inexpensive.

[0009] The use of water as the solvent in the inks of the invention reduces the toxicity of the inks compared to those including organic solvents. Further, the water based inks described are less volatile and hence can be applied to their substrates in a more controlled manner. In addition, the water based inks have lower toxicity than conventional inks and, as a result, fewer health and safety restrictions will be applied to the storage, handling and application of the water based inks. As organic solvents are not used, there is also a notable benefit in terms of storage. The water based cathode inks of the invention are less reactive, and so a wider range of containers may be used than can be utilised with known organic solvent based inks. Further, the use of water as a solvent provides an ink which is environmentally benign, as the water removed during formation of the inks into cathode layers can be returned to the environment.

I

100101 It will be understood, however, that whilst the inks of the invention are aqueous, and the primary solvent is water, low levels of organic solvents may be present if necessary to improve drying times, or ink stability. Where organic solvents are present, it is highly desirable that these be water miscible. Further, it is considered that the organic solvent would generally be present as no more than 10% of a solvent phase formed between the organic solvent and water (in the range 0 or 0.0001 -10%), ideally no more than 5% (in the range 0 or 0.000 1 -5%), often no more than 1 or 2 % (in the range 0 or 0.000 1 -1 or 2%). In many cases the organic solvent will be substantially entirely absent, so that the full benefits of the invention may be derived.

100111 The ceramic powder particles may be any ceramic powder particles known in the art, it is a distinct advantage of the invention that the cathode ink may be formulated from ceramic particles well known to the person skilled in the art, and which are readily available.

It is a further benefit that the ceramic powder particles of the invention may be chosen from a wide range of ceramic powder particles, providing for the formation of inks with a wide range of properties, which may therefore be used in a wide range of applications. However, it will often be the case that the ceramic powder particles will be selected from, lanthanum strontium manganate, nickel oxide, yttria-stabilised zirconia and combinations thereof Often the ceramic powder particles will be a combination of lanthanum strontium manganate and yttria-stabilised zirconia, although these ceramics may also be used alone, or in combination with other ceramic powder particles. Often a cathode layer formed from the ink of the invention will be a composite cathode as composite cathodes offer enhanced oxygen reduction in combination with good current collection capabilities. Where a composite cathode is desired, a combination of ceramic powder particles will be used; where at least one of the combination offers good current collection properties, and at least one of the combination good oxygen reduction capabilities. One such combination may be lanthanum strontium manganate and yttria-stabilised zirconia.

[0012] It will often be the case that the particle size of the ceramic powder particles will be selected such as to provide the best dispersion characteristics for the ink, in view of the need to disperse the ceramic powder particles in water. This would be clear to the person skilled in the art. However, it will often be the case that the particles will be of size (d50) in the range 0.01 iim -50 rim, in some cases, 0.1 im -30 Elm, 1 tm -10 m or 1 tm -5 rim. Where lanthanum strontium manganate is present, it will often have particles of size (d50) in the range 0.01.im -50 rim, in some instances in the range 0.1 tm -10 m or 1 tm -5 lm.

Where yttria-stabilised zirconia is used, it will often have particles of size in the range 0.1 lm -50 iim or 0.1 m-3OMmor lMm-S Elm.

[0013] Dispersion can also be aided by consideration of the surface area of the ceramic powder particles used, for instance, surface areas in the range 0.1 m2g' -50 m2g' can often provide stable ink formulations, often the surface area will be in the range 0.5m2g1 -22 m2g', particularly for particles of yttria-stabilised zirconia.

[0014] The dispersant of the invention is neutral. The dispersant provides for a substantially uniform dispersion of the ceramic powder particles throughout the ink. A wide variety of neutral dispersants may be used. This provides for a simple ink formulation.

Specifically, the use of a neutral dispersant (as opposed to an acidic or alkali dispersant) removes the need for the inclusion of a buffer, to control the pH of the ink. Alternatively, a neutralising step is not required as neutralising agent (i.e. an acid or alkali as appropriate) is not needed to return the pH to neutral or near neutral. Where acidic or alkaline dispersants are used in cathode ink formulations a buffer, or neutralising step, is necessary to prevent degradation of the ink during storage as a result of decomposition of the components of the ink in the acidic or alkaline conditions. However, the inclusion of a buffer or neutralising agent increases the cost of the ink formulation, and the complexity of the formulation parameters though the simple inclusion of a further chemically active component which may interact unpredictably with the other components of the ink affecting their chemical or physical behaviour. Finally, a neutral dispersant is easier to handle during ink formulation, as (unlike when acids or bases are being handled) no special safety precautions must be taken.

[0015] As used herein, the term "neutral dispersant" means a dispersant which, when in liquid form or aqueous solution has a pH measured in the range 6 -8, often 6.5 -7.5, often around 7. In other words, a liquid or solution which is not significantly acidic or basic, or which is neither acidic or basic.

[0016] In many cases the dispersant will comprise poly(N-vinylpynolidone) although any neutral dispersant which decomposes at temperatures above 800°C may be used.

Decomposition above this temperature is advantageous so that the sintered cathode layers formed from the ink contain only negligible residue of the dispersant. Where used, the poly(N-vinylpynolidone) of the invention will typically have an average (mean) molecular weight in the range 5000 -1,000,000, often in the range 10,000 -50,000, generally a range of 35,000 -45,000, often around 40,000. HypermerTM KD2 is another prefened dispersant, as are the other surfactants in the KD range (for instance KD 1, and KD3 available from ICI).

These dispersants are amine based cationic surfactants. Other possible commercial dispersants include SolsperseTM range of dispersants available from Lubrizol Advanced Materials Inc).

[0017] The cathode ink of the invention is intended for use in the formation of cathode layers, and hence cathodic electrodes. The cathode ink of the invention, once formed into a cathode layer is desirably capable of catalysing the decomposition of oxygen to oxide anions in a fuel cell, for instance a solid oxide fuel cell.

100181 In one example of an ink according to the invention, there is provided an ink comprising a dry component including 65% -99.9% ceramic powder particles and 0.1% - 10% dispersant. Often the ceramic powder particles will be present in the range 70% -90%; the dispersant in the range 2.5 -6%, generally in the range 3% -4.5%. The dry component is dispersed in water.

[0019] The ink may further comprise a binder, the binder being present to improve the stability of a cathode layer prepared from the ink, by improving the film forming properties of the ink, and enhancing the adhesion of a cathode layer formed from the ink to the substrate. As a result, the cathode layer formed is more robust. However, it should be noted, that a potential advantage of the invention is the ability to prepare cathode inks without binders; this has not been done before.

[0020] Any substance which can perform the function of a binder may be used, however, it is specifically envisaged that the binder may be selected from vinyl compounds, in particular polyvinyl compounds such as polyvinyl alcohol or polyvinyl butyral. Often, the binder will be polyvinyl alcohol. In some examples the binder will be present in the range 5% -20% of the composition, such that an example composition may comprise a dry component of 65% - 94.9% ceramic powder particles, 0.1% -10% dispersant and 5% -20% binder. In some instances the binder may be present in levels outside this range, if the nature of the ceramic powder particles is such that the desired film forming characteristics are not obtained.

However, typically binder levels will be in the range 5% -20% of the dry component, often 10% -15%.

[00211 It has been found that where binder and dispersant are present at levels significantly higher than those described above, that the ink formulation thickens unacceptably and cannot be applied to the substrate. Also, where the binder was present in levels significantly above the 20% suggested above, the cathode layer formed from the inventive ink was found to peel off the substrate to which it was applied. In addition, the ink became very slow to dry, resulting in running of the ink across the surface and hence in an uneven depth to the subsequently formed cathode layer.

100221 In a second aspect of the invention there is provided a cathode layer produced using the cathode ink of the first aspect of the invention. It is typical, where the cathode layer forms part of a solid oxide fuel cell, that the cathode layer be bonded to a solid electrolyte, such as yttria-stabilised zirconia. Alternatively, the cathode layer may be formed adjacent to and bonded to an interlayer which can be a mixture of the electrolyte material and the material forming the cathode layer. Such interlayers help to improve bonding of the cathode layer to the electrolyte. This reduces cracking of the fuel cell upon heating by providing adjacent layers of similar thermal coefficients.

[0023] It is desirable that the cathode ink be formulated to provide a cathode layer which has a similar thermal expansion coefficient to the electrolyte to remove the need for an interlayer. If the thermal expansion coefficient of the cathodic layer and the electrolyte are similar, cracking is reduced during thermal cycling, extending the lifetime of the fuel cell. In addition, this can improve conductivity of the cathodic layer to both electrons and ions.

Barrier layers can also be used to eliminate or reduce any unwanted side reactions between electrolyte and cathode material.

100241 The cathode ink may be applied using a variety of techniques including ink jet printing, dip-coating, screen printing, brush-painting, tape casting or aerosol deposition. The ink adheres to the electrolyte or interlayer surface and forms a cathode layer which catalyses the reaction in which oxygen is decomposed into oxide ions and conducts electrons.

100251 In a third aspect of the invention there is provided a solid oxide fuel cell comprising at least one cathode layer according to the second aspect of the invention. In some examples of the solid oxide fuel cell, two or more cathode layers are formed. Where two or more cathode layers are present, these may be of the same or different compositions. The use of cathode layers of different compositions, whether formed from inks of different compositions or whether one cathode layer is formed from an ink and another using alternative methods, is that the use of layers with different compositions, and hence different physical properties, provides for a wider range of solid oxide fuel cell constructions. For instance, a first layer may have excellent oxygen decomposition properties, but be of poor structural integrity and prone to damage in use. Such a layer could be combined with a second layer which, perhaps, has poorer oxygen decomposition properties, but which is robust, hence improving the overall structural integrity of a cathode prepared from the two cathode layers. Additionally or alternatively the two or more layers may have progressively changing properties, for instance the thermal expansion coefficient may change progressively with each layer as the distance from the electrolyte increases, the layer closest to the electrolyte having a thermal expansion coefficient most closely matching that of the electrolyte.

[00261 In a fourth aspect of the invention there is provided a process for forming an ink according to the first aspect of the invention, the process comprising the steps of: a. mixing the ceramic powder particles, and dispersant with water, and b. milling the ceramic powder particles, dispersant and water to stabilise the ink.

[00271 In a fifth aspect of the invention, there is provided a process for electrode formation comprising the steps of: a. depositing a cathode ink of the first aspect of the invention onto an electrolyte surface, and b. sintering the ceramic particles to form one or more cathode layers.

In such a process, the water evaporates during sintering, and the dispersant decomposes to leave a structure consisting essentially of sintered ceramic powder particles. The cathodic layer will therefore comprise a porous ceramic network. Sintering will often occur at a temperature in the range 800°C -1400°C, often 1000°C -1200°C.

100281 It will generally be the case that the electrode formed will be a cathode, and that the cathode be a composite cathode. The cathode will typically be used in a fuel cell.

[0029] In a sixth aspect of the invention there is provided a process for fuel cell formation comprising the steps of: a. providing an anode and electrolyte, b. forming an electrode using the process of the fifth aspect of the invention, and c. forming current connections. The fuel cell will generally be a solid oxide fuel cell such as a microtubular solid oxide fuel cell.

[00301 In a seventh aspect of the invention there is provided the use of a cathode ink according to the first aspect of the invention in the formation of an electrode for use in solid oxide fuel cells.

[00311 Unless otherwise stated each of the integers described in the invention may be used in combination with any other integer as would be understood by the person skilled in the art.

Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims.

[0032] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0033] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about".

Brief Description of the Drawing

[0034] In order that the present invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

100351 Figure 1 is a scanning electron microscope (SEM) secondary electron image showing a cross-section of the cell; [0036] Figure 2a is a secondary electron SEM image of the cell cross section and Figure 2b is a backscattered electron SEM image of the cell cross section of shown in Figure 2a; [00371 Figure 3 is a graphical representation of cathode ink performance showing current against voltage and power density curves for water based and solvent based cathode inks; [00381 Figure 4 is a graphical representation of current/voltage curves for a cathode ink of the invention; and [0039] Figure 5 is a graphical representation of current/voltage curves for a cathode ink of the invention, in the absence of binder.

Examples

Cathode Ink Preparation [0040] Cathode Ink compositions in accordance with Tables 1 and 2 below were made by the following method: * Weigh out solid components into a sealable vessel * Add the required volume of tap water (demineralised or distilled can also be used) * Add milling beads (beads of -1cm diameter were used) * Mill for 24 hours in a vibro-mill Table 1 -Cathode Ink Compositions Component Cathode ink 1 Cathode ink 2 LSM (Merck 0.82/0.18 -d50=1.2Rm) -20.OOg LSM (SSC 0.5/0.5 -d50=l.3Rm) 6.50g - YSZ (TZ-8Y -TOSOH Corp.) 6.50g -PVA l.56g 2.40g Water l6.OOml 14.00ml PYP (Average MW: 40,000) 0.48g 0.74g LSM = lanthanum strontium manganate YSZ = yttria-stabilised zirconia PVA = polyvinyl alcohol PYP = poly(N-vinylpyrrolidone) [004lJ Cathode inks 3 and 4 were prepared as for cathode inks 1 and 2 but without the presence of PVA.

Table 2 -Cathode Ink Compositions Component Cathode ink 5 Cathode ink 6 LSM (Merck 0.82/0.18 -d50=1.2Rm) -10.OOg LSM (SSC 0.5/0.5 -d50=l.3Rm) 3.25g - YSZ (TZ-8Y -TOSOH Corp.) 3.25g -KD2 0.13g 0.20g Acetone 8.OOml 7.OOml Glycerol Triolate Triolien 0. lOg -Terpinol (added after 24 hours) -1.OOg [0042j Cathode inks 7 and 8 were formed by substitution of the acetone in cathode inks 5 and 6 with water.

Cathode Ink Application [00431 A microtubular solid oxide fuel cell was prepared. The anode was a combination of yttria-stabilised zirconia with nickel. The tubes were made using a combination of yttria-stabilised zirconia with nickel oxide and the nickel oxide was then reduced to nickel during the cell preparation.

[0044] The electrolyte layer was pure yttria-stabilised zirconia and was coextruded with the anode material to form a tube suitable for use in a micro-tubular SOFC application.

[00451 Cathode ink 1 of Table 1 was applied over the electrolyte layer by brush coating.

Cathode Ink 1 contained LSM and yttria-stabilised zirconia to provide a cathode composite layer with a close match in thermal expansion coefficient to the electrolyte layer. A first cathode layer was then formed.

[0046] A second cathode layer was prepared from cathode ink 2 by application over the first cathode layer by brush coating. Cathode ink 2 contained LSM which acts as the catalyst in the fuel cell cathode reaction.

[0047] The cathode inks 1 and 2 were applied by brush painting. A first coat was applied using a smooth paint brush and allowed to air dry in a horizontal position for a period of 24 hours. A second coat was then applied using the same method.

[0048] The application of the cathode ink by brush coating was a suitable method for the production of a small test sample. For production of large numbers of cells the cathode ink could be applied by any conventional means such as dip-coating, inkjet printing, screen printing, brush-painting, tape-casting or aerosol deposition.

[00491 The development of dip-coating techniques facilitates the application of a number of thinner cathode layers, which increases the opportunity for using a range of ink compositions with a gradual reduction in the proportion of yttria-stabilised zirconia to give a smoother gradient between the electrolyte and cathode materials, thus reducing the possibility of thermal shock.

[0050] The cells were then sintered in an air atmosphere by heating to 500°C at 1.5°C/mm, then heating to 1150°C at 12.5°C/mm followed by a 2 hour hold at 1150°C and then cooling S to room temperature at a rate of 20°C/mm. The sintering cycle removed the solvent, binder and dispersant and fused the ceramic powder particles together to form a porous network structure. The tubes were then reduced in pure hydrogen at 750 °C for thirty minutes.

Cell Preparation 100511 The painted cells were then etched, silver-painted and silver wire-wrapped to provide cunent interconnections using methods known in the art. A high-temperature tolerant cement was then added to protect the partially-exposed anode before cell testing was performed.

Properties of Cathode Inks 1 and 2 100521 A Scanning Electron Microscope (SEM) was used to image a cross-section of the cell, showing the cathode layer clearly. Pictures were first taken using secondary electrons, to give an accurate picture of layer morphology and porosity, as shown in Figure 1. This figure shows the cathode 11, electrolyte 12 and anode 13. The total cathode layer thickness was found to be 30±10gm. The variation in thickness is likely to be due to the ink running slightly during drying. Rotating the cells during the drying period would reduce this variation.

[00531 Using Back-Scattered Electron Microscopy allowed the two separate layers of the cathode (the two inks) to be distinguished. Backscattered electrons are electrons from the initial beam which enter the sample, interact with an atom's nucleus inside the sample, then leave the sample. The larger the nuclei, the more backscattered electrons emitted. This was used to determine the elemental composition of the surface under study. The contrast in images produced by this technique is due to atomic numbers rather than sample morphology, and is also of lower resolution than pictures formed from secondary electron data. Figures 2a and 2b show secondary and backscattered electron pictures, respectively, for the same section of the cell. In addition to the anode 13 and electrolyte 12 these figures show two cathode layers 14 and 15, layer 14 comprising cathode ink 2, and 15 cathode ink 1. Although the information about surface shape is less accurate for Figure 2b, the colour contrast between the two cathode layers allows their approximate widths to be determined separately.

* Layer 1, which is adjacent to the electrolyte and deposited from the application of cathode ink 1, has a width of i3±5m.

* Layer 2, which is on the outside of the tube and deposited from the application of cathode ink 2 has a width of l7±7Rm.

[0054] A simple test rig was used for cell testing. The test rig consisted of a furnace, temperature controlling apparatus and a SolartronTM analytical 1400 Cell Test System connected to a computer where purpose-built programmes were run using Cell TestTM (v 5.2.0) software.

[0055] The cells were tested at 750°C with a flow rate of 2Oml/minute of hydrogen at 1.5 bar. Cunent-Voltage (IV) and power curves were plotted, as shown in Figure 3. Figure 3 shows that the recipe in Table 1 gave similar, but marginally improved, results as compared to known organic solvent based equivalents. When the PVP was replaced with that with a lower average molecular weight of 10,000, the performance was poorer, although it still showed the characteristic shape expected for an IV curve.

100561 The repeatability of these results was checked by running repeats on separate cells made in the same ways, with the same inks. Cells were also tested immediately after reaching temperature, and after a few hours of operation. Very good agreement was seen for all inks, and a sample of data for the best, water-based, ink is shown in Figure 4.

100571 The cells showed no degradation over the time of the test, and an improvement in performance was actually seen when comparing the initial IV curves to those taken two hours later. This suggests that some "pre-treatment" is useful -potentially the anode was not entirely reduced so the operating time at temperature can improve structure.

[0058] The performance of the cell manufactured using the water based inks was an improvement on the performance of a cell produced using equivalent cathode inks with an acetone carrier system in place of the water.

[0059] The performance of the cell produced using the water based cathode ink of the invention falls well within the range known in the prior art for organic solvent based cathode ink systems.

Properties of Cathode inks 3 and 4 [0060] A comparative test of cathode inks 1 and 2 was prepared, without the presence of the PVA binder. Tests clearly showed that a stable cathode ink could be prepared (Figure 5), with similar current-voltage properties to cathode inks containing binder.

Properties of Cathode inks 5 -8 [00611 A comparative test of two organic solvent based cathode inks (cathode inks 5 and 6) was made with water based inks 7 and 8. It was found that the water based inks performed as well as the organic based inks, but that water based inks 7 and 8 did not have the handling problems associated with organic solvent based inks.

100621 It should be appreciated that the inks, cathode layers, electrodes and fuel cells of the invention are capable of being incorporated in the form of a variety of embodiments, only a few of which have been illustrated and described above.

Claims

Claims 1. A cathode ink comprising ceramic powder particles dispersed in water, and a neutral dispersant.
2. An ink according to claim 1, further comprising a binder.
3. An ink according to any preceding claim, wherein the ceramic powder particles comprise lanthanum strontium manganate and yttria-stabilised zirconia.
4. An ink according to claim 3, wherein the lanthanum strontium manganate has particles of size in the range 0. lim -l0im.
5. An ink according to claim 3 or claim 4, wherein the yttria-stabilised zirconia has particles of size in the range 0. lim -30im.
6. An ink according to any of claims 3 to 5, wherein the yttria-stabilised zirconia has particles of surface area in the range 0.5m2g1 -22 m2g'.
7. An ink according to any preceding claim, wherein the dispersant decomposes at temperatures above 800°C.
8. An ink according to any preceding claim, wherein the dispersant comprises poly(N-vinylpyrrolidone).
9. An ink according to any of claims 2 to 8, wherein the binder comprises polyvinyl alcohol and/or polyvinyl butyral.
10. An ink according to any preceding claim, wherein the ink is capable of catalysing the decomposition of oxygen to oxide anions in a fuel cell.
11. An ink according to any preceding claim, comprising: a dry component including 65% -99.9% ceramic powder particles and 0.1% -10% dispersant, dispersed in water.
12. An ink according to claim 11, further comprising 5% -20% binder in the dry component.
13. A cathode layer produced using the cathode ink of any preceding claim.
14. A solid oxide fuel cell comprising at least one cathode layer according to claim 13.
15. A solid oxide fuel cell according to claim 14, wherein two cathode layers are formed.
16. A solid oxide fuel cell according to claim 15, wherein the cathode layers are of different compositions.
17. A process for forming an ink according to any of claims 1 to 12, comprising the steps of: a. mixing the ceramic powder particles, and dispersant with water; and b. milling the ceramic powder particles, dispersant and water to stabilise the ink.
18. A process for electrode formation comprising the steps of: a. depositing a cathode ink according to any of claims 1 to 12 onto an electrolyte surface; and b. sintering the ceramic particles to form one or more cathode layers.
19. A process according to claim 18, wherein the water evaporates during sintering.
20. A process according to claim 18 or claim 19, wherein the cathode is a composite cathode.
21. A process according to any of claims 18 to 20, wherein sintering occurs at a temperature in the range 800°C -1400°C.
22. A process according to any of claims 18 to 21 where the fuel cell is a solid oxide fuel cell.
23. A process according to claim 22 wherein the fuel cell is a micro-tubular solid oxide fuel cell.
24. A process for fuel cell formation comprising the steps of: a. providing an anode and electrolyte, b. forming an electrode using the process of any of claims 18 to 23, and c. forming current connections.
25. Use of a cathode ink according to any of claims 1 to 12 in the formation of an electrode for use in solid oxide fuel cells.
26. An ink, cathode layer, solid oxide fuel cell and process substantially as described herein with reference to the examples.