Fuel cell and method of operating fuel cell

A method of operating a fuel cell, comprising:

• detecting a first voltage of a first unit cell and a second voltage of a second unit cell, the first unit cell including a first membrane electrode assembly having a first electrolyte membrane and first electrodes provided on respective sides of the first electrolyte membrane, the second unit cell including a second membrane electrode assembly having a second electrolyte membrane and second electrodes provided on respective sides of the second electrolyte membrane;
• detecting a voltage difference between the first voltage and the second voltage;
• determining whether the voltage difference is out of a predetermined range;
• and
• adjusting, when the voltage difference is determined to be out of the predetermined range,
• humidity of a first reactant gas before being supplied to a first reactant gas passage which is provided in a first separator and through which the first reactant gas is to be supplied to one of the first electrodes, or
• humidity of a second reactant gas before being supplied to a second reactant gas passage which is provided in a second separator and through which the second reactant gas is to be supplied to another of the first electrodes.