WO1994022177A1

WO1994022177A1 - Solid oxide fuel cells

Info

Publication number: WO1994022177A1
Application number: PCT/GB1994/000479
Authority: WO
Inventors: Clive Richard Field
Original assignee: British Nuclear Fuels Plc
Priority date: 1993-03-13
Filing date: 1994-03-11
Publication date: 1994-09-29
Also published as: AU6211994A; GB9305189D0

Abstract

A solid oxide fuel cell construction or stack comprising an array of fuel cells each including an anode, a cathode and an electrolyte, a plurality of interconnect portions electrically connecting the anode and the cathode of adjacent cells, the fuel cells and the interconnect portions being formed in a unitary structure comprising a multiplicity of channels along which fuel and oxidant may be delivered in use, wherein the mutual configuration of the anodes, cathodes and interconnect portions is such that the fuel cells are electrically connected in a series chain across the cell construction transversely to the channels whereby in use an electrochemical voltage is developed across the series chain and wherein the interconnect portions are formed in structures which each comprise a closed or partially closed shape around at least 40 percent of the cross-sectional area of each such channel in the region where the interconnect material is provided.

Description

Solid Oxide Fuel Cells

The present invention relates to fuel cells in particular to solid oxide fuel cells.

Fuel cells are electrochemical devices that convert chemical energy obtained from the reactants into electrical energy. A number of different families of such devices have been developed in the prior art. These vary according to the type of electrolyte used in the cell and the usual temperature of operation. All of the devices consume fuel at the anode or negative electrode and consume an oxidant at the cathode or positive electrode. The present invention is concerned with so- called solid oxide fuel cells, herein called "SOFCs", in which the electrolyte is a solid refractory oxide, and which in operation employ a temperature of the order of 800 to 1000°C. Such cells are normally formed together in a stack whereby an additive voltage is formed by the stack.

In most known SOFCs the electrolyte is contained between an anode and a cathode and the anode and cathode of adjacent cells in a stack are connected by an interconnect or bipolar plate which permits electronic conduction between cells and allows reactant gases to be delivered separately to regions adjacent to the anode and cathode. The reactant gases will generally comprise oxygen usually supplied as air as oxidant and hydrogen or a hydrogen containing compound, eg hydrocarbon such as methane, as fuel. The interconnect or bipolar plate needs to be gas impervious to keep the reactant gases separate as well as electrically conducting to permit transport of electrons to and from the electrode surfaces to facilitate the electrochemical processes.

Prior patent specification GB 2175736A describes a solid oxide fuel cell stack comprising an array of fuel cell segments each comprising a unitary structure having a multiplicity of channels running between portions

SUBSTITUTESHEET{RULE26) defining anode, cathode and electrolyte zones. The channels provide passageways for the fuel and oxidant in use of the stack. The segments are electrically connected by interconnect portions to form a series chain running longitudinally along the stack, ie parallel to the channels, to develop an electrochemical voltage along the longitudinal chain. Such a construction has advantages over fuel cell structures which had been used or proposed previously but is complicated to manufacture since the interconnect portions have to connect the anode and cathode zones in adjacent segments and these zones are not in-line with one another. Furthermore, the structure is not easily suited to deal with the high thermally induced stresses experienced in use.

Prior specifications US 4816036, US 4499663 and GB 2148044A describe SOFC constructions in which triangular sided parallel channels bounded by cathode or anode materials run through a monoblock construction. In these constructions the interconnect portions between the cathodes and anodes are formed by simple plate like layers which run adjacent to one of the sides of the triangles in each set.

In US 4499663 and GB 2148044A the oxidant channels are closed at one end. The channels have a co-tubular construction whereby oxidant flows along the inner tube to the closed end and then back along the outer tube to the same end at which it is admitted.

The purpose of the present invention is to provide an improved SOFC construction whereby improved manufacturability, versatility of operation and behaviour under thermal and mechanical loading are offered.

According to the present invention there is provided a solid oxide fuel cell construction or stack comprising an array of fuel cells each including an anode, a cathode and an electrolyte, a plurality of interconnect portions electrically connecting the anode and the cathode of adjacent cells, the fuel cells and the interconnect portions being formed in a unitary structure comprising a multiplicity of channels along which fuel and oxidant may be delivered in use, wherein the mutual configuration of the anodes, cathodes and interconnect portions is such that the fuel cells are electrically connected in a series chain across the cell construction transversely to the channels whereby in use an electrochemical voltage is developed across the series chain and wherein the interconnect portions are formed in structures which each comprise a closed or partially closed shape around at least some of the channels enclosing at least 40 per cent of the cross-sectional area of each such channel in the region where the interconnect material is provided.

In the prior art configurations the interconnect material comprises, as noted above, simple planar plates which are adjacent to only one side of equilateral triangular channel shapes. We have found that by providing more structured interconnect portions in which the interconnect material is configured to bound more than one flat side of each channel shape in at least some of the channels, ie around more than one third desirably around 50 per cent or more of the inside surface of the channel in selected channels eg half of the channels in the parts of the channel where the interconnect material exists improved channel shapes giving better mechanical and thermal loading properties than in the prior art can be provided and such shapes can be manufactured in a more cost efficient manner than in the prior art as explained further below.

Desirably, in at least some of the channel regions where the interconnect material exists, the interconnect material extends around at least half of the channel walls or sides. The interconnect material may itself form the walls or sides or it may bound anode or cathode material forming the walls or sides. Desirably, the channels along which fuel and oxidant may be delivered in use all run parallel to one another.

Desirably, the channels are all open ended at both of their ends. Fuel and oxidant may be passed along the channels in the same direction or, more preferably, in countercurrent fashion. Alternate layers of channels in the series chain desirably have oxidant and fuel alternately passing along them.

The fuel cell construction according to the invention may comprise a plurality of the said series connected chains transverse to the said channels the plurality of chains being connected together in parallel.

The said channels may conveniently have a four sided, eg square or rectangular shape, eg in a square or rectangular grid. In such cases, the interconnect material may be a continuous structure extending across the cell construction and may be formed around three or four sides of the four sided channel shape of each channel.

More preferably, the channels may have a hexagonal shape. In this case, the interconnect material may be formed around at least three or six sides of the six sided channel shape of each channel. The hexagons of the hexagonal shaped channels may be regular or may be flattened hexagons (ie having a breadth to depth ratio > 1) wherein two facing sides are longer than the other sides. Alternatively, the hexagons may be chevron-shaped giving a re-entrant structure which allows the width of the structure to be substantially constant when the structure is loaded in that direction which will be the expected thermal loading direction and thereby reduce the effect of the thermal and the mechanical loading of the construction in use.

The cross-sectional shapes of the channels may be straight-sided quadrilaterals or hexagons although the shapes can alternatively be non-straight sided eg the channel cross-sectional sides may be curved.

Where the channels are hexagonal the hexagons are desirably formed in a honeycomb configuration whereby adjacent channels share common walls so that each channel at the interior of the construction shares its walls with six other channels.

Desirably, a series of continuous electrolyte layers are formed across the cell construction according to the present invention running transversely to the channels and extending in the same direction that the interconnect material extends. The electrolyte layers may be formed within wall regions of adjoining portions in which the channels are formed. The back-to-back wall surfaces of adjacent channels (facing inward into their respective channels) desirably comprise anode material and cathode material respectively, the anode and cathode material being separated by the electrolyte layer in the wall region. The anode material and the cathode material on the channel wall surfaces may be formed as layers on all of the wall surfaces of each channel. Alternatively, the anode material and cathode material may be formed only on the wall surface adjacent to the electrolyte layer whereby a series of continuous three-layer zones or structures each comprising an anode, cathode and electrolyte there between extend across the fuel cell construction transversely to the channels in the same sense as the interconnect material.

The continuous layers of electrolyte material may alternate with continuous layers of interconnect material as separating layers between channels having adjacent walls comprising anode and cathode materials. The continuous layers of these types of material may include planar layers but they can also be non-planar eg corrugated for example with a 'square or rectangular waveform¹ cross-section or where the channel sides are curved rather than planar with a wavy, ie sinuous cross- section. Where the continuous layers of interconnect material comprise planar layers the interconnect material may extend laterally of the continuous layers between sides of adjacent channel walls.

Alternatively or in addition, the electrolyte material may extend laterally of the continuous layers of electrolyte material, eg vertically if the layers run generally in a horizontal direction, between the walls of adjacent channels to improve the overall mechanical strength of the construction.

Desirably, layers of one or more of the different types of material employed in the cell construction according to the present invention, viz anode, cathode, electrolyte and interconnect material, are co-extruded (ie extruded together) as continuous layers. For example, the electrolyte and interconnect layers may be co-extruded (in a direction parallel to the channels being formed) the anode and cathode layers being subsequently deposited on the wall surfaces of the appropriate channels. Alternatively, in the case where continuous layers of all four materials extend across the cell construction, the anode, cathode, electrolyte and interconnect material may all be co-extruded. Co- extrusion has the advantage of simplifying and therefore reducing the cost of the manufacturing route. Generally, after formation of the materials eg by co-extrusion (by methods known for the extrusion or co-extrusion of ceramic materials) the layers formed are fired in a known way. Where different materials have been co-extruded the layers of such materials may be co-fired, ie fired together. Where layers of two of the materials are co- extruded and the layers of the other two materials are subsequently deposited, the first two materials are co- fired after extrusion before the other two materials are deposited. The other two materials are subsequently co- fired after deposition.

The various types of material employed in the fuel cell construction according to the present invention may be those known to be suitable in the prior art. Thus, the following are suitable examples:

(i) electrolyte : yttria stabilised zirconia

(ii) cathode : lanthanum magnanite doped, eg with strontium, to provide electrical conductivity; or urania doped with a rare earth metal oxide;

(iii) anode : nickel and/or cobalt zirconia cermet doped, eg yttria-stabilised; or urania doped with a rare earth metal oxide;

(iv) interconnect : lanthanum chromite, doped eg with strontium to provide electrical conductivity; or urania doped with a rare earth metal oxide.

The interconnect material may alternatively comprise a cermet (ceramic-metal matrix) using adjacent anode and cathode layers as protective surfaces. The interconnect material, either in metallic or cermet or other form, may be extruded as a powder in a binder, eg by co-extrusion with the other materials. The powder may then be consolidated subsequently during co-firing.

As in the prior art, the respective layers of material may be from 0.002 cm to 0.01 cm thick. The layers of anode material and cathode material particularly may be from 0.002 cm to 0.05 cm thick.

In use, known fuel and oxidant gases are delivered to and removed from the appropriate channels by known means, eg an end cap or a manifold structure, at each end of the fuel cell construction, wherein all appropriate channels are connected to one source or one removal exit (as appropriate) of each gas. Fuel, eg hydrogen or methane, is directed to the channels having wall surfaces coated with anode material and oxidant, eg air, is directed to the channels having wall surfaces coated with cathode material.

The manifold structure may for example be a ceramic piping structure which may be a ceramic moulding. In one example, the piping structure may have a series of rows of inlet gas orifices and a series of rows of outlet gas orifices. The inlet gas orifices may connect each of the channels in a row of channels required to receive inlet gas with an inlet branch pipe running transversely to the channels, each of the branch pipes being connected to a common inlet gas pipe. Likewise, the outlet orifices may connect each of the channels in a row of channels from which spent gas is required to be removed with an outlet branch pipe running transversely to the channels, each of the branch pipes being connected to a common outlet gas pipe.

In use, the reactant gases are pre-heated (as in the prior art) to an appropriate temperature, typically 900C, before being introduced into the fuel cell construction according to the present invention. The cathode, anode, interconnect and electrolyte materials provided in the cell construction according to the present invention in one of the ways described above need not extend along the entire length of the gas delivery channels. A region of each such channel at both ends of the channel may be free of such materials whereby the heat from exhaust gases may be more efficiently exchanged with incoming gas to maintain the temperature of such gas. Such materials may be present respectively at different stages along the length of the channels.

Hydrogen is the active part of the fuel. If pure hydrogen is employed as the fuel then it is only necessary to heat the gas before it passes over the anode and cathode part of the cell. If hydrocarbons are used it is necessary to reform the hydrocarbon, eg with steam in the presence of a catalyst into carbon dioxide and hydrogen. The source of steam can be external or obtained from recycling the exhaust gas which contains steam (as a result of the oxidation of hydrogen) back into the fuel inlet stream. In the case of the honeycomb stack the electrode free regions of the channels described above may be used for reformation as well as preheating of gases prior to passing over the anode and cathode part of the stack. It is also possible to use a more complex end cap construction to allow the passage of steam from the exhaust gas flow to the fuel inlet gas flow. This may be achieved using two channel stacks in a single layer of channels the fuel being passed in different directions along the two stacks so that the outlet gas flow is bled to the inlet gas flow. An outlet pipe may have bulb shaped portions which include an orifice to permit this bleeding. The bulb shape facilitates use of the Bernoulli Principle to effect the bleeding.

Two outer walls of the cell construction are desirably coated with continuous conducting layers or plates extending longitudinally in the same sense as the channels. Such layers or plates serve as terminals whereby the current and voltage developed by the cell construction or stack can be applied to a load in an external circuit. For example, the conducting layers may be parallel layers having the integral structure forming the cells formed in a sandwich structure there between.

The fuel cell construction according to the present invention has the advantage over the prior art that it affords an improved cell construction in terms of thermal and mechanical properties and may be manufactured by a simpler and cheaper route desirably involving co- extrusion and co-firing. Once an extrusion die has been produced quantities of honeycomb or other cellular structures can be extruded with ease and speed. By employing co-extrusion in the manufacture of a fuel cell construction according to the present invention a further important advantage obtained is that the requirement is reduced for discrete seals to the ends of the extruded construction where it connects to the inlet and outlet gas manifolds or end caps. A still further advantage of the invention is that the fuel cell construction is more fault-tolerant than the prior art longitudinal constructions. Discontinuities in the interconnect portions of the transverse construction of the present invention can be more readily accommodated electrically by the parallel nature of the interconnect.

The cellular channel structure in a monolithic structure reduces the effect of the high thermal stresses experienced in use and these stresses can be further reduced by arranging that adjacent channels in a given row are offset (ie not in-line) relative to one another. For example, in the case of a hexagonal honeycomb structure the individual channel walls rather than a continuous planar wall structure can be allowed to bend to accommodate the strain. Conveniently, a high aspect ratio (ie breadth to depth ratio >1) can be employed for the cell channel shapes thereby increasing the number of channels per unit stack of cells and thereby reducing the amount of material and cost to give a particular power output.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a transverse cross-sectional view of a fuel cell construction;

Figure 2 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 3 is an illustration of the electrical equivalent of the constructions shown in Figures 1 and 2; Figure 4 shows the construction of Figure 1 or of Figure 2 fitted with external terminals to form an external circuit power source;

Figure 5 is a longitudinal section of the construction shown in Figure 1 or in Figure 2 on the line V-V, the structure in this case having end caps fitted.

Figure 6 is a transverse section on the line VI-VI of one the end caps shown in Figure 5;

Figure 7 is a transverse cross-sectional view of a fuel cell rectangular grid construction;

Figure 8 is a transverse cross-sectional view of an alternative fuel cell rectangular grid construction;

Figures 9 and 10 are cross-sectional views of alternative fuel cell rectangular grid constructions.

Figure 11 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 12 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 13 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 14 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 15 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 16 is a transverse cross-sectional view of an alternative fuel cell construction;

Figure 17 is a vertical side sectional elevation through a stack of layers of alternate fuel and oxidant channels and an associated end cap arrangement;

Figure 18 is a sectional end elevation on the line 18-18 in Figure 17;

Figure 19 is a sectional plan view on the line 19-19 in Figure 17;

Figure 20 is a section plan view on the line 20-20 in Figure 17. In the following examples, the terms "vertical" and "horizontal" refer to the directions as illustrated in the drawings. In practice, the cell construction is not limited to being in a particular orientation relative to the horizontal and vertical axes of the environment or location in which it is used.

As shown in Figure l a fuel cell construction 1 comprises an integral or monolithic multitube arrangement wherein the tubes together form a regular hexagonal honeycomb configuration comprising a multiplicity of channels la, lb. Each wall of the channels la, lb of the honeycomb configuration is formed of interconnect material 3 except that after every two channels in a vertical direction the horizontal channel walls and the walls connecting such horizontal walls are formed in a three-layer structure 6 comprising anode material 5, cathode material 7 and electrolyte material 9 therebetween. Each three-layer structure 6 extends as a continuous structure or layer horizontally across the overall construction 1. Thus, interconnect material bounds three sides of every channel la, lb.

Channels la are bounded by the anode material 5 and in use fuel is delivered along such channels. Channels lb are bounded by the cathode material 7 and in use oxidant is delivered along such channels.

In the manufacture of the construction 1 shown in Figure 1 the entire cell construction or monoblock is co- extruded in a direction parallel to the axes of the channels la, lb and then co-fired.

Figure 2 shows an alternative honeycomb fuel cell construction 1. Items having the same reference numerals as in Figure 1 have the same functions. In the case of Figure 2 the anode material 5 coats every wall of each of the channels la and the cathode material 7 coats every wall of each of the channels lb. The interconnect material 3 is provided on to form each side of channel la and lb. In the case of the Figure 2 construction the interconnect material 3 and the electrolyte material 9 are co-extruded in a direction parallel to the channels la, lb to form the basic honeycomb structure (construction 1) with layers of electrolyte material 9 at intervals of two channels vertically formed in the horizontal channel walls and in the inclined channel walls joining the horizontal channel walls. The extruded construction is subsequently co-fired after which the anode material 5 is then deposited on the walls of the cell la and the cathode material 7 is then deposited on the walls of the cell lb. The anode material 5 and cathode material 7 is subsequently co-fired.

The construction 1 shown in Figure 2 requires more manufacturing steps than that shown in Figure 1 but has a simpler co-extrusion step because not so many layers need to be co-extruded.

In use, as in Figure 1, fuel and oxidant are respectively delivered along the channels la, lb shown in Figure 2.

Figure 3 illustrates the electrical circuit equivalent of the cell construction 1 shown in Figure 1 or Figure 2 (repeated in less detail as Figure 3(a)). The three-layer structure 6 at each channel wall forms a fuel cell element E and, as shown in Figure 3(b), the elements E are connected serially by the interconnect material 3 in a vertical sense. Adjacent elements E are also connected in parallel in a horizontal sense by the interconnect material 3.

Figure 4 shows how electrical energy is extracted from the construction 1 of Figure 1 or of Figure 2 (again shown in simplified form) . An upper conducting plate 11 acting as a positive terminal is fitted across the interconnect material 3 so as to run generally parallel to the three-layer structures 6 and a lower conducting plate 13 acting as a negative terminal is fitted parallel thereto. A sandwich structure is thereby formed by the construction 1 between the plates 11, 13. A load 15 in an output circuit 17 may be electrically connected across the terminals 11, 13.

Figure 5 shows a longitudinal section of the construction 1 of Figure 1 or of Figure 2. The flow of oxidant 0 and fuel F takes place in alternate opposite directions along the adjacent channels la, lb as indicated by the arrows labelled 0 and F respectively. End caps 19, 21 are fitted at the respective ends of the construction 1 to act as manifolds to enable the gases to be introduced to and extracted from the channels la, lb. The anode material 5, the cathode material 7 and the electrolyte material 9 need not extend the entire length of the channels la, lb between the end caps 19, 21. For example, these materials may not be present beyond a limit XI on the left hand side and beyond a limit X2 on the right hand side of the construction (as shown in Figure 5) to facilitate gas heat exchange as described above.

Figure 6 shows a transverse cross-section through the end cap 19. As seen, the cap 19 incorporates two interfitting comb-shaped pipe arrangements. A first arrangement includes a vertical gas inlet pipe 23a having horizontally extending branches 23b. A second arrangement includes a vertical pipe 25a having horizontally extending branches 25b. The branches 23b connect with the channels lb (at right angles thereto) at gas connections 23c and the branches 25b connect with the channels la (at the right angles thereto) at gas connections 25c. Oxidant is thereby introduced to the channels lb via the pipe 23a, branches 23b and connections 23c and spent fuel is removed from the channels la via the connections 25c, the branches 25b and the pipe 25a. Oxidant and fuel are kept separated by the construction of the end cap 19. The end cap 21 (Figure 5) has a similar construction but the direction of flow of the oxidant and fuel gas there is in the opposite sense.

Figure 7 shows a monolithic fuel cell construction analogous to that of Figure 1 but containing rectangular channels 21a and 21b (respectively functioning like the channels la, lb of Figure 1 respectively) in a multitube grid 21. The grid 21 is formed of interconnect material 23 except that the channel walls which are horizontal between channels la, lb at two channel intervals in a vertical sense comprise a continuous three layer structure 26 which includes an anode material 25, a cathode material 27 and an electrolyte material 29 there between.

The construction shown in Figure 7 is manufactured and operated in a manner similar to that of the construction shown in Figure 1.

Figure 8 shows a monolithic fuel cell construction or grid 1 analogous to that of Figure 2 and similar to that shown in Figure 7. In this case, items as in Figure 7 are given the same reference numerals. The anode material 25 in this case coats every wall of each of the channels 21a and the cathode material 27 coats every wall of each of the channels 21b. The construction shown in Figure 8 is manufactured and operated in a manner similar to the construction shown in Figure 2.

Further alternative constructions similar to those shown in Figure 7 and Figure 8 are shown in Figures 9 and 10. Items similar to those in Figures 7 and 8 again have the same reference numerals.

In Figure 9 adjacent channels la, lb when considered along a given horizontal row are offset in a vertical sense relative to one another. Each three-layer structure 26 comprising the anode material 25, the cathode material 27 and the electrolyte material 29 therefore undulates as shown in Figure 9. The structure is manufactured and operated in a manner similar to that shown in Figure 1.

In Figure 10 the anode material 25a coats each wall of the channels 21a and the cathode material 27a coats each wall of the channels 21b. The structure may be manufactured and operated in a manner similar to that shown in Figure 2. The cathode material 27 and the anode material 25 may be deposited and co-fired separately after co-extrusion and co-firing of the interconnect material 23 electrolyte material 29.

Further alternative structures analogous to those shown in Figures 9 and 10 (not shown) may be formed wherein the channels la, lb are offset relative to one another in both a horizontal and a vertical sense.

Figure 11 shows a further alternative version of a honeycomb fuel cell construction 1. Items having the same reference numerals as in Figure 1 have the same functions. In the example shown in Figure 11 the amount of interconnect material 3 employed is reduced compared with that shown in Figure 1. This reduces the problem of the mechanical strength of the construction relying mainly on the strength of the interconnect material 3. In this case, the interconnect material 3 is formed in corrugated layers spaced two channels apart in a vertical sense which separate channels lb bounded by cathode material 7 and cells la bounded by anode material 5. Electrolyte material 9 provides similar separation layers alternately between the separation layers provided by the layers of interconnect material 3. However, the electrolyte material 9 also extends between the channel walls of adjacent channels both bounded by anode material 5 (adjacent channels la) and between the channel walls of adjacent channels both bounded by cathode material 7 (adjacent channels lb) to provide added strength.

Figure 12 shows an example analogous to that shown in Figure 11 but with rectangular rather than hexagonal channels. Thus, again layers of interconnect material 3 spaced two channels apart in a vertical sense run horizontally forming vertical separations between walls bounded by anode material 5 forming channels la and cell walls bounded by cathode material 7 forming channels lb. Layers of electrolyte material 9 spaced two channels apart form similar separation layers alternately with the layers of interconnect material 3. However, in order to give strength between interconnect material 3 layers, the electrolyte material 9 also extends vertically between adjacent walls of channels bounded by anode material 5 and between adjacent walls of channels bounded by cathode material 7.

Figure 13 shows a further example of a cell construction 1 analogous to that shown in Figure 11. Items having like reference numerals have similar functions. The regular hexagonal shaped channels of Figure 11 providing the channels la and lb are replaced in Figure 13 by hexagons which are chevron shaped in which two adjacent sloping sides SI and S2 of the hexagon in each channel form a joint which points inward toward the centre of the hexagon. Horizontally running corrugated layers of interconnect material 3 and of electrolyte material 9 are provided in a similar manner to that in Figure 11, the electrolyte material 9 also extending between the adjacent walls of channels la bounded by anode material 5 and adjacent walls of channels lb bounded by cathode material 7.

Figure 14 shows a further cell construction alternative to that shown in Figure 13. Items having like reference numerals have like functions. Again the channels are chevron shaped except that in Figure 14 the interconnect material 3 rather than the electrolyte material 9 extends between walls of adjacent channels la bounded by anode material 5 and walls of adjacent channels lb bounded by cathode material 7. Figure 15 shows a further cell construction alternative (but similar to) to that shown in Figure 13. Items having like reference numerals have like functions. In the case of Figure 15 the channels are chevron-shaped but the chevrons have curved sides rather than straight sides.

Figure 16 shows a further cell construction alternative to that shown in Figure 15. Items having like reference numerals have like functions. In the case of Figure 16 the channels are four-sided. Three of the sides are curved and a fourth (horizontal as shown in Figure 16) is straight and is adjacent to a straight side of the electrolyte material 9. The curved sides C opposite to the straight sides in each channel are either concave sides SI or convex sides S2. The interconnect material 3 runs adjacent to the various sides SI and S2.

Figures 17 to 20 show an alternative end cap and oxidant and fuel flow arrangement. As seen in Figure 17 air as oxidant and fuel are passed along channels (lb and la respectively) of the kind described in Figure 1 in alternate Layers L_& and Lp in the arrangement. In each layer L^ air is passed in one direction along some channels and in an opposite direction along other channels. Likewise, in each layer Lp fuel is passed in one direction along some channels and in an opposite direction along other channels. An end cap 41 has therefore at each layer L^ both air inlets Al for some channels and air outlets AO for other channels and at each layer Lp both fuel inlets FI for some channels and fuel outlets FO for other channels.

The inlets FI and Al and the outlets FO and AO are shown together in an end sectional view in Figure 18.

Figure 19 shows a horizontal section through an air channel layer L^ and the adjacent part of the end cap 41. Air is passed from right to left as seen in Figure 19 along half of the channels to air outlets AO and is extracted via an outlet manifold pipe 43 formed within the end cap 41. Air is passed from left to right as seen in Figure 19 along the other half of the channels from air inlets Al. It is delivered to the inlets Al via an inlet manifold pipe 45 formed within the end cap 41.

Figure 20 shows a horizontal section through a fuel channel layer Lp and the adjacent part of the end cap 41. Fuel is passed from left to right along certain channels and from right to left along other channels. Admitted fuel is passed from an inlet manifold pipe 47 through the fuel inlets FI. Exhaust from spent fuel is passed to an outlet manifold pipe 49 through the fuel outlets FO. The pipes 47 and 49 are formed as part of the end cap 41. The outlet pipe 49 has a series of bulb shaped portions 50 having in their wall a small orifice 51 which communicates with the inlet pipe 47. By this construction of the end cap 41 steam in the exhaust gas is allowed to bleed into the incoming fuel stream to pre¬ heat the incoming fuel. This assists the conversion of hydrocarbon fuel into elemental hudrogen. The bulbous shape of the portions 50 enables the bleeding to be facilitated into the higher speed fuel inlet stream by the Bernoulli Principle.

Claims

1. A solid oxide fuel cell construction or stack comprising an array of fuel cells each including an anode, a cathode and an electrolyte, a plurality of interconnect portions electrically connecting the anode and the cathode of adjacent cells, the fuel cells and the interconnect portions being formed in a unitary structure comprising a multiplicity of channels along which fuel and oxidant may be delivered in use, wherein the mutual configuration of the anodes, cathodes and interconnect portions is such that the fuel cells are electrically connected in a series chain across the cell construction transversely to the channels whereby in use an electrochemical voltage is developed across the series chain and wherein the interconnect portions are formed in structures which each comprise a closed or partially closed shape around at least 40 per cent of the cross- sectional area of each such channel in the region where the interconnect material is provided.

2. A fuel cell construction as in Claim 1 and wherein the channels along which fuel and oxidant may be delivered in use all run parallel to one another.

3. A fuel cell construction as in Claim 1 or Claim 2 and wherein the fuel cell construction comprises a plurality of the said series connected chains transverse to the said channels the plurality of chains being connected together in parallel.

4. A fuel cell construction as in any one of the Claims 1 to 3 and wherein the said channels have a square or rectangular shape or a hexagonal shape the sides of the shape being straight sided or curved.

5. A fuel cell construction as in Claim 4 and wherein the channels are hexagonal and the hexagons are formed in a honeycomb configuration whereby adjacent channels share common walls.

6. A fuel cell construction as in any one of the preceding Claims and wherein a series of continuous electrolyte layers are formed across the cell construction running transversely to the channels, the electrolyte layers being formed within wall regions of adjoining portions in which the channels are formed.

7. A fuel cell construction as in Claim 6 and wherein the back-to-back wall surfaces of adjacent channels, facing inward into their respective channels, comprise anode material and cathode material respectively, the anode and cathode material being separated by the electrolyte layer in the wall region.

8. A fuel cell construction as in Claim 7 and wherein the anode material and the cathode material on the channel wall surfaces are formed as layers on all of the wall surfaces of each channel.

9. A fuel cell construction as in Claim 7 and wherein the anode material and cathode material are formed only on the wall surface adjacent to the electrolyte layer whereby a series of continuous three-layer zones or structures each comprising an anode, cathode and electrolyte therebetween extend across the fuel cell construction transversely to the channels.

9. A fuel cell construction as in any one of the preceding Claims and wherein layers of one or more of the different types of material employed in the cell construction according to the present invention, viz anode, cathode, electrolyte and interconnect material, have been co-extruded.

10. A fuel cell construction as in any one of the preceding Claims and wherein one or more of the different types of material which have been co-extruded have, after co-extrusion, been co-fired.

11. A fuel cell construction as in any one of the preceding Claims and wherein an end cap or a manifold structure is fitted to the construction at each end of the fuel cell construction, wherein all appropriate channels are connected to one source of fuel gas and one source of oxidant gas as appropriate and whereby spent gases may be removed.

12. A fuel cell construction as in Claim 11 and wherein each manifold structure is a ceramic piping structure having a series of rows of inlet gas orifices and a series of rows of outlet gas orifices, the inlet gas orifices connecting each of the channels in a row of channels required to receive inlet gas with an inlet branch pipe of the end cap running transversely to the channels, each of the branch pipes being connected to a common inlet gas pipe, and the outlet orifices connecting each of the channels in a row of channels from which spent gas is required to be removed with an outlet branch pipe running transversely to the channels, each of the branch pipes being connected to a common outlet gas pipe.

13. A fuel cell construction as in Claim 11 or Claim 12 and wherein a region of the channels at both ends of the fuel cell construction adjacent to the end caps is electrode-free to permit heat exchange between spent and incoming gases.

14. A fuel cell construction as in any one of the preceding Claims and further including continuous conducting layers or plates coating outer walls of the construction and extending longitudinally in the same sense as the channels, the layers or plates serving as terminals whereby the current and voltage developed by the cell construction or stack can be applied to a load in an external circuit.

15. A construction as in Claim 14 and wherein the conducting layers are substantially parallel layers having the integral structure forming the cells formed in a sandwich structure therebetween.