NOVELTY - Operating a fuel cell involves providing fuel cell system comprising oxygen removing device having inlet fluidly connect to reactant gas sources and outlet of cathode flow field and outlet capable of fluidly communicating with anode and cathode gas flow fields; directing gas flow from reactant gas source and outlet of cathode flow field to oxygen removing device for forming oxygen free gas stream; configuring anode control valve to fluidly connect outlet of oxygen removing device to inlet of anode gas flow field; and purging anode and cathode gas flow field with the stream.    USE - For operating a fuel cell (claimed).    ADVANTAGE - By removing residue oxygen in the cathode gas flow field by purging the method eliminates possible chemically reactive gases and potential electrochemical reactions on the cathode while the anode gas flow field is still filled with a fuel gas. Particularly the simultaneous purging and sequential purging processes eliminate or significantly reduce possible decay of fuel cell performance during fuel cell startup and shutdown. During fuel cell startup and shutdown, fast change-over of gases in the anode and cathode gas flow fields minimizes possible and undesirable side reactions that results in degradation of fuel cell electrodes and fuel cell performance.    DETAILED DESCRIPTION - Operating a fuel cell involves providing a fuel cell system comprising a fuel cell stack having a cathode gas flow field, an anode gas flow field, a reactant gas source fluidly connected to an inlet of the cathode gas flow field through a cathode control valve, a fuel gas source fluidly connected to an inlet of the anode gas flow field through an anode control valve, an oxygen removing device having an inlet fluidly connect to at least one of the reactant gas source and an outlet of the cathode flow field and an outlet capable of fluidly communicating with each of the anode gas flow field and the cathode gas flow field; providing flow of a reactant gas through the oxygen removing device from the reactant gas source and/or the outlet of the cathode gas flow field to remove any oxygen gas component, thus producing oxygen free gas stream at the outlet of the device; configuring the anode control valve to fluidly connect the outlet of the oxygen removing device to the inlet of the anode gas flow field; purging the anode gas flow field with the oxygen free gas stream so that the anode gas flow field is filled with the oxygen free gas; and purging the cathode gas flow field with the oxygen free gas stream so that the cathode gas flow field is free of oxygen gas. The oxygen removing device comprises an oxygen removing medium configured to effectively contact the reactant gas flowing through the device and to selectively adsorb, absorb, or chemically react with oxygen gas to remove oxygen gas from the reactant gas. The fuel cell system further comprises a gas humidity control device fluidly connected to the outlet and/or inlet of oxygen removing device. An INDEPENDENT CLAIM is included for the fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows an exemplary flow chart illustrating a fuel cell operation process.