Fuel cell system, comprising - a first fuel cell unit having at least one first fuel cell and at least one second fuel cell, - a second fuel cell unit having at least one third fuel cell and at least one fourth fuel cell, - a hydrogen tank, - an oxygen supply unit, - at least one oxidant supply unit, - a control unit coupled with the first fuel cell unit and the second fuel cell unit, - at least one air inlet, - at least one inert gas outlet (31, 33), - at least one exhaust gas switching unit, and - a water outlet, wherein the first fuel cell unit and the second fuel cell unit are operable independently, wherein each of the first fuel cell, second fuel cell, third fuel cell and fourth fuel cell comprises a hydrogen inlet coupled with the hydrogen tank, wherein each of the first fuel cell, second fuel cell, third fuel cell and fourth fuel cell comprises an oxidant inlet, wherein the oxidant inlets of the first fuel cell and the fourth fuel cell are coupled with the at least one air inlet wherein the oxidant inlets of the second fuel cell and the third fuel cell are coupled with the at least one oxidant supply unit, said oxidant supply unit is coupled with the oxygen supply unit and the at least one air inlet, and wherein each of the first fuel cell, second fuel cell, third fuel cell and fourth fuel cell comprises an exhaust outlet wherein the exhaust outlets of the first fuel cell and of the fourth fuel cell are coupled with the at least one inert gas outlet (31, 33) wherein the exhaust outlets of the second fuel cell and of the third fuel cell are coupled with the at least one exhaust switching unit, wherein said exhaust switching unit is coupled with the at least one inert gas outlet (31, 33) and the water outlet, wherein the control unit is adapted for controlling the operation of the first fuel cell unit and the second fuel cell unit, which operation comprises: a. the operation of either the first fuel cell unit or the second fuel cell unit in a first mode of operation, in which either at least one of the first fuel cell and the second fuel cell or at least one of the third fuel cell and the fourth fuel cell of the respective fuel cell unit (4, 6) is operated by the exclusive supply of air and hydrogen; b. the operation of either the first fuel cell unit or the second fuel cell unit in a second mode of operation, in which either only the second fuel cell or only the third fuel cell of the respective fuel cell unit (4, 6) is operated by the supply of oxygen through the oxidant supply unit and hydrogen without the supply of air; and c. the simultaneous operation of the first fuel cell unit and the second fuel cell unit in a third mode of operation, in which at least one of the first fuel cell and the second fuel cell and at least one of the third fuel cell or the fourth fuel cell in the respective fuel cell unit (4, 6) is operated by the exclusive supply of air and hydrogen. - a first fuel cell unit having at least one first fuel cell and at least one second fuel cell, - a second fuel cell unit having at least one third fuel cell and at least one fourth fuel cell, - a hydrogen tank, - an oxygen supply unit, - at least one oxidant supply unit, - a control unit coupled with the first fuel cell unit and the second fuel cell unit, - at least one air inlet, - at least one inert gas outlet (31, 33), - at least one exhaust gas switching unit, and - a water outlet, wherein the first fuel cell unit and the second fuel cell unit are operable independently, a. the operation of either the first fuel cell unit or the second fuel cell unit in a first mode of operation, in which either at least one of the first fuel cell and the second fuel cell or at least one of the third fuel cell and the fourth fuel cell of the respective fuel cell unit (4, 6) is operated by the exclusive supply of air and hydrogen; b. the operation of either the first fuel cell unit or the second fuel cell unit in a second mode of operation, in which either only the second fuel cell or only the third fuel cell of the respective fuel cell unit (4, 6) is operated by the supply of oxygen through the oxidant supply unit and hydrogen without the supply of air; and c. the simultaneous operation of the first fuel cell unit and the second fuel cell unit in a third mode of operation, in which at least one of the first fuel cell and the second fuel cell and at least one of the third fuel cell or the fourth fuel cell in the respective fuel cell unit (4, 6) is operated by the exclusive supply of air and hydrogen.