NOVELTY - A polymer electrolyte member (PEM )fuel cell power plant has fuel cell assemblies. Each of the fuel cell (2, 4) assemblies comprises a polymer electrolyte membrane (6) member; a cathode reactant flow field plate (16) which is impermeable to liquids; an anode reactant flow field plate (14) which is permeable to liquids so that water can move through the anode reactant flow field plate; and coolant passages (12). The coolant passages contain a water coolant which saturates the anode reactant flow field plate and is operative to evaporatively cool the fuel cell assemblies during operation.    USE - For use in powering vehicles.    ADVANTAGE - By providing a single water permeable plate for each cell, the amount of water present in the power plant at shut down is limited to a degree which does not require adjunct water purging components to remove water from the plates when the power plant is shut down during freezing ambient conditions. This limits the amount of residual ice in the power plant that forms in the plates during shut down in such freezing conditions. The power plant can thus be restarted and brought up to full operating power levels quickly due to the reduced amount of ice that must be melted during startup. The power plant has low thermal mass. During shut down periods at ambient freezing or nonfreezing temperatures, there will be no reactant gas cross over from one cell to another since the water permeable reactant flow field plate will contain sufficient quantities of water or ice to form a seal, and thus be impermeable to gases, and the other reactant flow field plate in each cell will be impermeable to gases while containing negligible quantities of water.    DETAILED DESCRIPTION - The PEM fuel cell power plant has fuel cell assemblies. Each of the fuel cell assemblies comprises a polymer electrolyte membrane member; a cathode reactant flow field (8) plate adjacent to one side of the electrolyte membrane member, the cathode reactant flow field plate being substantially impermeable to liquids; an anode reactant flow field (10) plate adjacent to an opposite side of the electrolyte membrane, the anode reactant flow field plate being permeable to liquids so that water can move through the anode reactant flow field plate; and coolant passages disposed adjacent to the anode reactant flow field plate. The coolant passages contain a water coolant which saturates the anode reactant flow field plate and is operative to evaporatively cool the fuel cell assemblies during operation.    INDEPENDENT CLAIMS are also included for the following:    (1) limiting the thermal mass of ice in a PEM fuel cell power plant assembly during freezing temperature shut down of the fuel cell power plant assembly and preventing reactant gas cross over from one fuel cell to adjacent fuel cells in the power plant assembly; and    (2) limiting the amount of an aqueous coolant in a PEM fuel cell assembly during shut down of the fuel cell assembly.    DESCRIPTION OF DRAWING(S) - The figure is a cross sectional view of two adjacent fuel cell assemblies in a fuel cell power plant.    Fuel cell (2, 4)    Polymer electrolyte membrane (6)    Cathode reactant flow field (8)    Anode reactant flow field (10)    Coolant passages (12)    Anode reactant flow field plate (14)    Cathode reactant flow field plate (16)