A typical fuel cell has a structure in which a membrane-electrode assembly (MEA) is sandwiched by separators, each having a flow path for supplying raw materials, the MEA having a fuel electrode catalyst layer provided on one surface of an electrolyte membrane and an oxidant electrode catalyst layer provided on the other surface of the electrolyte membrane so that the two catalyst layers face each other across the electrolyte membrane, and further having diffusion layers provided outside the respective catalyst layers sandwiching the electrolyte membrane therebetween, and such structure serves as one unit, i.e., a so-called unit cell. A common fuel cell system uses a cell stack having a plurality of such unit cells stacked therein so as to obtain a desired amount of electric power. Electric power is generated by supplying raw materials, such as hydrogen and oxygen (hereinafter also referred to as source gases or reactant gases), to each catalyst layer.
When generating electric power at the fuel cell by using hydrogen as a fuel gas to be supplied to the fuel electrode and using air as an oxidant gas to be supplied to the oxidant electrode, hydrogen produces hydrogen ions and electrons at the fuel electrode. The produced electrons travel through an external terminal and external circuit and reach the oxidant electrode. At the oxidant electrode, water is produced from: oxygen included in the supplied air; hydrogen ions that have passed through the electrolyte membrane; and electrons that have reached the oxidant electrode through the external circuit. Through these electrochemical reactions occurring at the fuel electrode and the oxidant electrode, the fuel cell functions as an electric cell.
In many fuel cell systems, an output drawn from the fuel cell is not constant. The fuel cell has a voltage varying according to an output drawn from the fuel cell, and the fuel cell voltage decreases when the output drawn from the fuel cell increases. If an oxide film formed on the surface of the oxidant electrode catalyst is reduced upon a decrease of the voltage, a metallic component (e.g., platinum supported on carbon) in the catalyst will be eluted when the fuel cell voltage again increases (when the output drawn from the fuel cell decreases).
To respond to the above, Patent Document 1, indicated below, proposes a fuel cell system aimed at suppressing the elution of catalysts, such as platinum, occurring due to changes in the output of the fuel cell. The fuel cell system disclosed in Patent Document 1 comprises an output control means for supplying electric power to a motor from a fuel cell and from a secondary battery and controlling the output of the fuel cell based on the SOC (State of Charge) of the secondary battery. If the SOC of the secondary battery is greater than 10%, the output control means limits the output of the fuel cell so that the fuel cell voltage does not go below a predetermined voltage. It is stated that this fuel cell system can suppress degradation caused by the elution of catalysts (e.g., platinum) in the oxidant electrode due to changes in the fuel cell voltage.