TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Fuel Cell System: The first and second plates (30, 30', 30''; 32, 32', 32'') at opposite ends of a fuel cell (14) are formed of a material having a first relative hardness, and the key member (60, 60', 60'') is formed of a material having a second relative hardness equal to or less than the first relative hardness. The fuel cells (14) include an interfacial seal (34) in mutual engagement with the first and the second plates (30, 30', 30''; 32, 32', 32''), the interfacial seal being formed of a material having a third relative hardness less than both the first and the second relative hardness. The key member (60, 60', 60'') is a water-resilient polymer. The first and the second seats (62, 62', 62''; 64, 64', 64'') respectively recessed in the engagement surfaces of the first and the second plates (30, 30', 30''; 32, 32', 32'') of the first and the second fuel cells (14) have respective cross-sectional geometries and the key member (60, 60', 60'') has a cross-sectional geometry adapted for firm engagement with the sides of the first and the second seats when the fuel cell stack (12) is fully assembled. Prior to assembly of the fuel cell stack (12), the cross-sectional geometry of the key member (60, 60', 60'') is slightly smaller than the cross-sectional geometry of the first and the second seats (62, 62', 62''; 64, 64', 64'') and the length of the key member is slightly greater than the combined depths of the first and the second seats, the key member being longitudinally compressed and expanded transversely into the firm engagement with the sides of the first and the second seats when the fuel cell stack is fully assembled. At least one (62, 64, 62'') of the first and the second seats (62, 62', 62''; 64, 64', 64'') is an annular recess having a central mounting stub (70, 70''), and the key member (60, 60'') is annular and is mountedly disposed on the mounting stub for assembly. The first and the second seats (62', 64') are circular recesses and the key member (60') is a disk. The first and the second plates (30, 30', 30''; 32, 32', 32'') of each fuel cell (14) are electrically conductive. The electrolyte (40) between the first and the second plates (30, 30', 30''; 32, 32', 32'') of a respective fuel cell (14) comprises a proton exchange membrane. The slip mitigation arrangement (50, 50', 50'') is located between each pair of the fuel cells (14) in the fuel cell stack (12), where the slip mitigation arrangement (50, 50', 50'') between each pair of the fuel cells is multiple in number (150, 250, 350, 450). The slip mitigation arrangement (50, 50', 50'') between the pair of the fuel cells (14) is multiple in number (150, 250, 350, 450).