A PEM fuel cell power plant system (10) has a process air pump (26), which
may be a fan, a blower or a compressor, with an adiabatic efficiency of
between 40% and 70%. The process air at the inlet 27 of the cathode
reactant gas flow field 16 is between 1.07 atmospheres and 1.85
atmospheres, and may be at an optimal pressure for maximum overall system
efficiency P={0.45+2.6E-1.8E.sup.2} atms.+-.0.2 atms where P is the air
inlet pressure and E is the adiabatic efficiency of the process air pump.