Fuel cell system

A fuel cell system employing a fuel cell stack having an air electrode, an electrolyte membrane, and a fuel electrode, the fuel cell system comprising:

• an air residual space, from which air supplied to the air electrode is not discharged to stay behind therein when the fuel cell system is stopped, the air residual space communicating with the air electrode;
• a hydrogen residual space, from which hydrogen supplied to the fuel electrode is not discharged to stay behind therein when the fuel cell system is stopped, the hydrogen residual space communicating with the fuel electrode;
• and
• a control unit configured to control whether air is to be supplied to the air residual space and whether air is to be discharged from the air residual space, and control whether hydrogen is to be supplied to the hydrogen residual space and whether hydrogen is to be discharged from the hydrogen residual space,
• wherein a residual ratio corresponds to a ratio of the molecular number of hydrogen that stays behind in the hydrogen residual space to the molecular number of oxygen in air that stays behind in the air residual space when the fuel cell system is stopped,
• wherein when the residual ratio is greater than an upper limit of a reference range, the control unit performs a first control to supply air to the air residual space or a second control to discharge hydrogen from the hydrogen residual space at least until the residual ratio reaches the upper limit, and
• wherein when the residual ratio is less than a lower limit of the reference range, the control unit performs a third control to discharge air from the air residual space or a fourth control to supply hydrogen to the hydrogen residual space at least until the residual ratio reaches the lower limit.