a fuel cell system includes a fuel cell stack that receives a cathode feed gas and has an exhaust stream and a heat transfer stream flowing therefrom . a charge - air heat exchanger enables heat transfer between the heat transfer stream and the cathode feed gas . the charge - air heat exchanger also enables heat transfer between the heat transfer stream and the cathode feed gas to compensate for the adiabatic cooling effect . furthermore , the charge - air heat exchanger vaporizes the liquid water to provide water vapor . the water vapor humidifies the cathode feed gas .