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A fuel cell stack comprising multiple arrays of one or more fuel cells, each comprising an electrolyte layer, an anode layer and a cathode layer; gas separator plates between adjacent fuel cells; and oxidant gas distribution passages and fuel gas distribution passages between adjacent fuel cells; an

A fuel cell stack comprising multiple arrays of one or more fuel cells, each comprising an electrolyte layer, an anode layer and a cathode layer; gas separator plates between adjacent fuel cells; and oxidant gas distribution passages and fuel gas distribution passages between adjacent fuel cells; and gas separators opening to the cathode layers and the anode layers, respectively, of the fuel cells. The fuel cell arrays comprise at least first stage fuel cell arrays having associated first fuel gas distribution passages to receive fuel gas from one or more fuel gas supply manifolds and second stage fuel cell arrays having associated second fuel gas distribution passages which receive fuel exhaust from the fuel cells of the first stage fuel cell arrays. The second stage fuel cell arrays are interleaved in the stack between first stage fuel cell arrays to improve thermal gradients. Other interleaving arrangements are possible.

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1. A fuel cell assembly comprising: a stack of multiple fuel cell arrays, each fuel cell array comprising one or more fuel cells and each fuel cell comprising an electrolyte layer, an anode layer on one side of the electrolyte layer and a cathode layer on an opposite side of the electrolyte layer, s

1. A fuel cell assembly comprising: a stack of multiple fuel cell arrays, each fuel cell array comprising one or more fuel cells and each fuel cell comprising an electrolyte layer, an anode layer on one side of the electrolyte layer and a cathode layer on an opposite side of the electrolyte layer, said stack further comprising fuel gas distribution passages which open to the anode layers of the fuel cells in the stack;one or more fuel supply manifolds for supplying fuel gas to fuel gas distribution passages in the stack;the fuel cell arrays comprising at least first stage fuel cell arrays having associated first fuel gas distribution passages connected to the one or more fuel gas supply manifolds to receive fuel gas directly therefrom and second stage fuel cell arrays having associated second fuel gas distribution passages arranged to receive fuel exhaust from the fuel cells of the first stage fuel cell arrays for supply of said fuel exhaust to the fuel cells of the second stage fuel cell arrays in the stack;wherein the anode layers of the fuel cells of the first stage fuel cell arrays and/or the first fuel gas distribution passages comprise steam reforming catalyst for steam reforming hydrocarbon in the fuel gas contacting said anode layers and/or the first fuel gas distribution passages;wherein from 18 to 36% of the fuel cell arrays in the stack are second stage fuel cell arrays and there is a greater proportion of first stage fuel cell arrays than second stage fuel cell arrays; andwherein at least one of the first and second stages of fuel cell arrays comprises fuel cell arrays that are disposed in the stack between fuel cell arrays of the other of the first and second stages of fuel cell arrays. 2. A fuel cell assembly according to claim 1, wherein an inlet side of the second fuel gas distribution passages opening to the anode layer of each fuel cell of the second stage fuel cell arrays is connected directly to an exhaust side of the first fuel gas distribution passages opening to the anode layers of at least two fuel cells of the first stage fuel cell arrays. 3. A fuel cell assembly according to claim 1, wherein at least one fuel manifold is provided between an inlet side of the second fuel gas distribution passages and an exhaust side of the first fuel gas distribution passages to supply fuel exhaust from plural first fuel gas distribution passages to plural second fuel gas distribution passages. 4. A fuel cell assembly according to claim 3, wherein said at least one fuel manifold comprises a fuel exhaust outlet manifold connected to a fuel exhaust inlet manifold whereby fuel exhaust from the first fuel gas distribution passages passes in one direction through the fuel exhaust outlet manifold and in an opposite direction along the fuel exhaust inlet manifold to the second fuel gas distribution passages. 5. A fuel cell assembly according to claim 1, wherein each fuel cell array of the one of the first and second stages of fuel cell arrays is disposed adjacent at least one fuel cell array of the other of the first and second stages of fuel cell arrays. 6. A fuel cell assembly according to claim 5, wherein each second stage fuel cell array is disposed in the stack between a respective two first stage fuel cell arrays. 7. A fuel cell assembly according to claim 1, wherein in at least part of the stack there is a repeating pattern of three first stage fuel cell arrays and one second stage fuel cell array. 8. A fuel cell assembly according to claim 1, wherein in at least part of the stack there is a repeating pattern of four first stage fuel cell arrays and one second stage fuel cell array. 9. A fuel cell assembly according to claim 1, wherein there is a repeating pattern of arrangement of first stage fuel cell arrays and second stage fuel cell arrays throughout the stack. 10. A fuel cell assembly according to claim 1, wherein there is a greater number of second stage fuel cell arrays in a first half of the stack than in a second half of the stack. 11. A fuel cell assembly according to claim 1, wherein there is a smaller proportion of second stage fuel cell arrays in a central half of the stack than in each end quarter of the stack. 12. A fuel cell assembly according to claim 11, wherein the proportion of second stage fuel cell arrays in each end quarter of the stack is at least twice as great as in the central half of the stack. 13. A fuel cell assembly according to claim 1, wherein the number of first stage fuel cell arrays in the stack is in the range of 64 to 82% of the total number of fuel cell arrays in the stack. 14. A fuel cell assembly according to claim 1, wherein all of the second fuel gas distribution passages are connected to at least one fuel exhaust manifold for directing fuel exhaust from the second fuel gas distribution passages exteriorly of the stack or for recycling to the stack. 15. A fuel cell assembly according to claim 1, wherein the fuel cell arrays further comprise at least one third stage fuel cell array having associated third fuel gas distribution passages arranged to receive fuel exhaust from fuel cells of the second stage fuel cell arrays for supply of said fuel exhaust to the fuel cells of the at least one third stage fuel cell array in the stack. 16. A fuel cell assembly according to claim 15, wherein at least one fuel manifold is provided between an inlet side of the third fuel gas distribution passages and an exhaust side of the second fuel gas distribution passages to supply fuel exhaust from the second fuel gas distribution passages to the third fuel gas distribution passages. 17. A fuel cell assembly according to claim 15, wherein, as a proportion the total number of first, second and third stage arrays of fuel cells in the stack, the first stage arrays comprise 50 to 70%, the second stage arrays comprise 31 to 23%, and the third stage arrays comprise 19 to 7%. 18. A fuel cell assembly according to claim 15, wherein each third stage fuel cell array is disposed in the stack adjacent at least one first stage fuel cell array. 19. A fuel cell assembly according to claim 18, wherein each third stage fuel cell array is disposed in the stack between two first stage fuel cell arrays. 20. A fuel cell stack according to claim 18, wherein all of the second stage fuel cell arrays are disposed towards a first end of the stack and all of the third stage fuel cell arrays are disposed towards a second end of the stack. 21. A fuel cell assembly according to claim 1, further comprising a steam extractor connected between the fuel gas distribution passages of a final stage of fuel cell arrays and the fuel gas distribution passages of a penultimate stage of fuel cell arrays to condense steam from the fuel exhaust received from the fuel gas distribution passages of the penultimate stage of fuel cell arrays prior to supply of said fuel exhaust to the fuel gas distribution passages of the final stage of fuel cell arrays.