NOVELTY - A fuel cell system (10) comprises a stacked adjacent fuel cells (14) collectively forming a fuel cell stack (12), the fuel cells each including a pair of first and second plates (30, 32) and an electrolyte (40); and a slip mitigation arrangement (50, 50', 50'') comprising a first seat (62, 62',62''), a second seat (64, 64', 64''), and a key member (60, 60', 60'').    USE - The fuel cell system is used to convert a source fuel into an electric current.    ADVANTAGE - The present invention provides fuel cell systems and related arrangements for limiting relative motion between fuel cells in a fuel cell stack.    DETAILED DESCRIPTION - A fuel cell system (10) comprises a stacked adjacent fuel cells (14) collectively forming a fuel cell stack (12), the fuel cells each including a pair of first and second plates (30, 32) at their respective opposite ends and an electrolyte (40) between them, a first the fuel cell (14) having a first the plate (30, 30', 30'') in engagement with a second the plate (32, 32', 32'') of a second fuel cell (14) adjacent to the first fuel cell at their respective engagement surfaces; and a slip mitigation arrangement (50, 50', 50'') between at least one pair of the first and the second fuel cells comprising: a first seat (62, 62',62'') recessed in the engagement surface of the first plate (30, 30', 30''), a second seat (64, 64', 64'') recessed in the engagement surface of the second plate (32, 32', 32''), and a key member (60, 60', 60'') having opposite ends seated in the first and the second recessed seats such that relative movement between the first and the second fuel cells is reduced. An INDEPENDENT CLAIM is a method for minimizing slip due to shear loading between adjacent fuel cells (14) in a stack (12) of plural fuel cells, the fuel cells each including a pair of first and second plates (30, 30', 30''; 32, 32', 32'') at their respective opposite ends and an electrolyte (40) between them, a first the fuel cell having a first the plate (30, 30', 30'') for engagement with a second the plate (32, 32', 32'') of a second fuel cell adjacent to the first fuel cell at their respective engagement surfaces, the method comprises providing a first seat (62, 62',62'') recessed in the engagement surface of the first plate; providing a second seat (64, 64', 64'') recessed in the engagement surface of the second plate; the first and the second seats having respective cross-sectional geometries; providing a key member (60, 60', 60'') disposed in one of the first and the second seats prior to stack assembly, the key member having a cross-sectional geometry similar to but slightly smaller than the cross-sectional geometries of the first and the second seats and having a length slightly greater than the combined depths of the first and the second seats; and assembling and longitudinally compressing the fuel cells in the stack to bring the fuel cells into engagement with one another and correspondingly longitudinally compress the key member and cause its transverse expansion into firm engagement with the sides of the first and second seats to thus reduce any movement between the fuel cells.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a fuel cell system.    Fuel cell system (10)    Fuel cell stack (12)    Fuel cell (14)    First and second plates (30, 32)    Interfacial seal (34)    Electrolyte (40)    Slip mitigation arrangement (50)