Systems and methods are provided for using fuel cell staging to reduce or minimize variations in current density when operating molten carbonate fuel cells with elevated CO.sub.2 utilization. The fuel cell staging can mitigate the amount of alternative ion transport that occurs when operating molten carbonate fuel cells under conditions for elevated CO.sub.2 utilization.

A method of controlling a fuel cell device includes: a step of determining whether impedance between a fuel electrode and an oxidant electrode is greater than a predetermined threshold during a steady operation of the fuel cell device; a step of decreasing a flow rate of a gas circulating via a circulation passage connecting a gas introduction passage of the gas supply unit to a gas discharge passage when the impedance is greater than the predetermined threshold; a step of measuring the voltage between the fuel electrode and the oxidant electrode when the impedance is equal to or less than the predetermined threshold, and determining whether the voltage is equal to or less than a first predetermined threshold; and a step of increasing the flow rate of the gas circulating via the circulation passage when the voltage is equal to or less than the first predetermined threshold.

An object is to readily provide an output limit of a fuel cell by using temperature of a cooling medium, while improving the startability of a fuel cell. When a cell voltage obtained from an end-portion cell of a fuel cell is equal to or lower than a first threshold value, a controller of a fuel cell system sets an output limit amount used for output limit of the fuel cell to be smaller than an output limit amount according to temperature of a cooling medium measured by a temperature measurement unit.

A method for controlling a fuel cell system includes supplying an oxidant gas, via an oxidant gas supply channel, to a cathode electrode of a fuel cell. The oxidant gas is moistened to be supplied to the cathode electrode with a humidifier. An opening degree of a bypass channel valve is decreased via a feedforward control to a first opening degree when an impedance has reached a lower limit value. The opening degree of the bypass channel valve is increased via the feedforward control to a second opening degree when the impedance has reached an upper limit value. The first opening degree is smaller than the second opening degree.

A method for repairing a fuel cell stack having a plurality of individual cells involves the steps of: a) identifying at least one degraded individual cell in the fuel cell stack; and b) deactivating the at least one degraded individual cell.