Fuel cell system and method of operating fuel cell system

A fuel cell system comprising:

• a fuel cell, including a fuel gas flow passage and an oxidant gas flow passage, configured to generate an electric power with supply of the fuel gas to the fuel gas flow passage and the oxidant gas to the oxidant gas flow passage;
• fuel gas supplying unit for supplying the fuel gas into the fuel gas flow passage;
• an oxidant gas supplying unit for supplying the oxidant gas into the oxidant gas flow passage;
• an oxidant gas supplying flow passage, extending from the oxidant gas supplying unit to the oxidant gas flow passage, through which the oxidant gas flows;
• an oxidant off-gas discharge flow passage through which the oxidant off-gas discharged from the oxidant gas flow passage flows;
• a diluter, installed in the oxidant off-gas discharge flow passage, configured to dilute the fuel off-gas discharged from the fuel gas flow passage with the oxidant off-gas;
• a branched gas flow passage configured to connect the oxidant gas flow passage or the oxidant off-gas discharge flow passage upstream from the diluter to the diluter and allow a branched gas to flow toward the diluter;
• a pressure controlling unit configured to control a pressure of the branched gas;
• an OCV determining unit configured to determine whether an OCV of the fuel cell is equal to or greater than a predetermined OCV at a system startup;
• and
• an I-V characteristic decreasing unit configured to start generation of the electric power in the fuel cell after the OCV determining unit determines that the OCV of the fuel cell is equal to or greater than the predetermined OCV and decreasing an I-V characteristic of the fuel cell by decreasing a stoichiometric ratio of the oxidant gas,
• wherein the pressure controlling unit decreases the pressure of the branched gas introduced into the diluter when the I-V characteristic of the fuel cell is decreased by the I-V characteristic decreasing unit.