Patent ID: 7125617

Claim:
A method of operating a phosphoric acid fuel cell having an assembly including an anode, a cathode, and an electrolyte layer impregnated with phosphoric acid and interposed between said anode and said cathode, said method comprising the steps of: before generating electric energy from said phosphoric acid fuel cell, supplying a fuel gas and an oxygen-containing gas to said cathode to raise the temperature of said assembly to an operating temperature with heat generated by a reaction between hydrogen contained in said fuel gas and oxygen contained in said oxygen-containing gas at said cathode; and before generating electric energy from said phosphoric acid fuel cell, causing said hydrogen and said oxygen to react with each other at said cathode under a condition in which the concentration of the phosphoric acid at the time the amount of water generated by the reaction and the amount of water evaporated from the phosphoric acid impregnated in said electrolyte layer are in equilibrium is equal to or greater than a reference phosphoric acid concentration which is capable of keeping the phosphoric acid fuel cell at a predetermined performance level.