The present invention provides a gas generator that may be used for startup and shutdown of a fuel cell. In one non-limiting embodiment, the gas generator may include a nitrogen generator structured to receive air, extract oxygen (O2) from the air and discharge the balance in the form of a nitrogen-rich gas; a merging chamber structured to receive a hydrocarbon fuel and the nitrogen-rich gas and to discharge a feed mixture containing both the hydrocarbon fuel and the nitrogen-rich gas; and a catalytic reactor structured to receive the feed mixture and to catalytically convert the feed mixture into a reducing gas.