Patent ID: 7517600

Claim:
A fuel cell system comprising: a fuel cell stack receiving a cathode inlet airflow and outputting a cathode exhaust gas flow; a compressor for providing the cathode inlet airflow to the stack; a water vapor transfer device receiving the cathode inlet airflow from the compressor and the cathode exhaust gas flow from the fuel cell stack, said water vapor transfer device using water vapor in the cathode exhaust gas to humidify the cathode inlet air; and a controller for controlling the relative humidity of the cathode exhaust gas flow so as to control the relative humidity of the cathode inlet air, said controller causing a cathode of the fuel cell stack to operate in a first cathode pressure range based on the current density being produced by the stack if the stack current density is below a predetermined value, and causing a cathode of the fuel cell stack to operate in a second cathode pressure range based on the current density being produced by the stack if the stack current density is above the predetermined value, wherein the first and second pressure ranges are different and the second pressure range is narrower than the first pressure range.