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A fuel cell stack has at least two segments of fuel cells each having reactant gas passages. Each of the cells in each segment is arranged such that the reactant gas passages of each cell are in parallel with each other cell. Flow of fuel cell fluids, normally in a gaseous state on the anode and cat

A fuel cell stack has at least two segments of fuel cells each having reactant gas passages. Each of the cells in each segment is arranged such that the reactant gas passages of each cell are in parallel with each other cell. Flow of fuel cell fluids, normally in a gaseous state on the anode and cathode side of each cell, is in a gravity assisted downward direction. Gravity assisted flow directs water formed in each cell to lower removal points of the stack segments. Each pair of segments is separated by a separator segment having a separator channel, the separator segment forming an integral unit of the stack. Each separator channel redirects the entire flow of each fluid within the stack from the bottom of an upstream segment to the top of a next or downstream segment, without reacting the fluid, controlling relative humidity between stack segments.

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1. A fuel cell stack comprising:a plurality of fuel cells separated into at least two fuel cell segments each having a portion of said plurality of fuel cells disposed therein and arranged as successive upstream and downstream pairs of segments;each of said portion of fuel cells of each segment havi

1. A fuel cell stack comprising:a plurality of fuel cells separated into at least two fuel cell segments each having a portion of said plurality of fuel cells disposed therein and arranged as successive upstream and downstream pairs of segments;each of said portion of fuel cells of each segment having a plurality of reactant gas passages provided with a common inlet reactant gas passage and a common outlet reactant gas passage;each pair of segments having a separator segment disposed between said upstream segment and said downstream segment, said separator segment in fluid communication with said common outlet reactant gas passage of said upstream segment and said common inlet reactant gas passage of said downstream segment of the same segment pair; andeach of the plurality of reactant gas passages of said at least two fuel cell segments being disposed in a unidirectional gravity assisted flow direction. 2. The fuel cell stack of claim 1 further comprising:each pair of said at least two fuel cell segments being arranged as adjacent segments separated by said separator segment, said separator segment forming a contiguous structural connection between each pair of said at least two fuel cell segments; andeach separator channel having a generally upward oriented flow passage providing fluid communication between said common outlet reactant gas passage of said upstream segment and said common inlet reactant gas passage of said downstream segment. 3. The fuel cell stack of claim 2, wherein said plurality of reactant gas passages comprises a group of anode passages and a group of cathode passages. 4. The fuel cell stack of claim 3, further comprising:said group of anode passages being in fluid communication with a stack anode inlet port through a preselected one of said common inlet reactant gas passages; andsaid group of anode passages being in fluid communication with a stack anode outlet port through a preselected one of said common outlet reactant gas passages. 5. The fuel cell stack of claim 4, further comprising:said group of cathode passages being in fluid communication with a stack cathode inlet port through a preselected one of said common inlet reactant gas passages; andsaid group of cathode passages being in fluid communication with a stack cathode outlet port through a preselected one of said common outlet reactant gas passages. 6. The fuel cell stack of claim 5, wherein each passage of said group of cathode passages is disposed in a cross flow relationship with a counterpart passage of said group of anode passages. 7. The fuel cell stack of claim 5 wherein each passage of said group of cathode passages is disposed in a parallel flow relationship with a counterpart passage of said group of anode passages. 8. The fuel cell stack of claim 3 further comprising a group of coolant passages being in fluid communication with a stack coolant inlet port through a common inlet coolant passage, said group of coolant passages being in fluid communication with a stack coolant outlet port. 9. The fuel cell stack of claim 8, wherein each passage of said group of coolant passages is disposed adjacent to and in a parallel flow relationship with a counterpart passage of at least one of said group of anode passages and said group of cathode passages. 10. The fuel cell stack of claim 1 wherein a quantity of fuel cells within each segment varies between each of said at least two fuel cell segments. 11. The fuel cell stack of claim 1 wherein a quantity of fuel cells within each segment decreases between each segment from an inlet portion of the stack to an outlet portion of the stack. 12. A fuel cell stack comprising:a plurality of fuel cell assemblies each including an anode catalyst on a first face and a cathode catalyst on a second face;a plurality of anode gas passages, each said passage disposed adjacent to a respective one of said first face of said plurality of fuel cell assemblies;a plurality of cathode gas passages, each said passag e disposed adjacent to a respective one of said second face of a respective one of said plurality of fuel cell assemblies;said plurality of fuel cell assemblies being arranged in at least two pairs of adjacent segments of cells wherein said plurality of anode gas passages and cathode gas passages of said plurality of fuel cell assemblies are provided in a unidirectional gravity assisted flow direction within each said adjacent segment of said stack; andeach of said at least two adjacent segments having at least one separator segment disposed there between defining a generally upward flow passage for connecting the plurality of said anode gas passages and said cathode gas passages of said adjacent segments. 13. The fuel cell stack of claim 12 further comprising:an anode gas separator channel disposed between each said pair of adjacent segments in fluid communication with the anode gas passages within each said pair of adjacent segments; anda cathode gas separator channel disposed between each said pair of adjacent segments in fluid communication with the cathode gas passages within each said pair of adjacent segments. 14. The fuel cell stack of claim 13 wherein a coolant passage is disposed between each of said first and second faces of any two consecutive fuel cells of said plurality of fuel cell assemblies. 15. The fuel cell stack of claim 14 wherein a coolant separator channel is disposed between each said pair of adjacent segments in fluid communication with the coolant passages of the fuel cell assemblies within each said pair of adjacent segments. 16. The fuel cell stack of claim 12 wherein water formed in each said segment of cells is discharged to at least one stack drain. 17. A fuel cell stack comprising:a plurality of fuel cell assemblies each including an anode catalyst on a first face and a cathode catalyst on a second face;a plurality of anode gas passages, each said passage disposed next to said first face of a respective one of said plurality of fuel cell assemblies;a plurality of cathode gas passages, each said passage disposed adjacent to said second face of a respective one of said plurality of fuel cell assemblies;said plurality of fuel cell assemblies being arranged in at least two adjacent segments of cells wherein at least one each of said anode gas passages and said cathode gas passages are provided in series between said segments and the at least one of said anode gas passages and said cathode gas passages are disposed in a unidirectional gravity assisted flow direction within each segment of said stack; andeach of said anode and cathode gas passages within at least one pre-selected segment having a geometry varied to control a pressure drop of said anode and cathode gas passages. 18. The fuel cell stack of claim 17 wherein a number of fuel cell assemblies within each segment decreases between segment pairs from an inlet portion of the stack to an outlet portion of the stack. 19. The fuel cell stack of claim 18, further comprising:each said segment pair being one of an upstream segment and a downstream segment;each said segment of each said segment pair having a pressure drop; andsaid pressure drop of said downstream segment being one of equal to and less than said pressure drop of said upstream segment for each segment pair from the inlet portion of the stack to the outlet portion of the stack. 20. The fuel cell stack of claim 19, wherein said pressure drop of said downstream segment of at least one segment pair is decreased by providing at least one of an increased fuel cell assembly width for each fuel cell assembly of said downstream segment, and a decreased assembly length for each fuel cell assembly of said downstream segment. 21. A method to operate a fuel cell stack having a plurality of fuel cells each having a plurality of reactant gas passages extending therebetween, and a cascaded stack design providing individual segments of differing numbers of fuel cells, said method comprising the steps o f:arranging said segments to form adjacent pairs of segments each including a plurality of fuel cells with a varying quantity of fuel cells in each segment to vary a stoichiometry of each segment;orienting all of the reactant gas passages of said plurality of fuel cells in a unidirectional gravity assisted flow direction within each said segment of said stack;disposing at least one separator channel between each said segment pair; andconnecting the plurality of reactant gas passages between each said segment pair by said at least one separator channel wherein said reactant gas passages in each said segment are connected in parallel and said reactant gas passages of each said segment are connected in series to an adjacent segment. 22. The method of claim 21 further comprising the further steps of:collecting a water volume from both the plurality of fuel cells and each separator channel; anddischarging the water volume to at least one stack drain.