Method for operating a fuel cell system in a mode of reduced power output

1. A method for operating a fuel cell system in a normal mode and a mode of reduced power output, wherein the fuel cell system comprises a fuel cell stack (BS) having at least one fuel cell (BZ) with an anode (A), a cathode (K), a proton exchange membrane, an anode fuel inlet, a cathode oxidant inlet, an anode outlet, a cathode outlet, a hydrogen supply and an oxidant supply, wherein the oxidant is air, the method comprising:

interrupting the air supply to the cathode (K) side of the fuel cell during the changeover from the normal mode to the mode of reduced power output, and

reducing the electric voltage (U) of the fuel cell stack (BS) by application of a current pulse.

2. The method according to claim 1 , wherein the electric voltage (U) of the fuel cell stack (BS) is regulated by means of an adjustable electric load (KP) in such a manner, that it is within a working region limited by an upper and a lower voltage limit value.

3. The method according to claim 1 , wherein the electric voltage (U) of the fuel cell stack (BS) is regulated by a DC-DC transformer.

4. The method according to claim 1 , wherein the electric voltage (U) of the fuel cell stack (BS) is regulated by an adjustable electric load (KP).

5. The method according to claim 1 , wherein the hydrogen supply to the anode (A) is not interrupted during the changeover to the mode of reduced power.

6. The method according to claim 1 , wherein depleted cathode air (KA) is again returned to the cathode (K) via a cathode recirculation circuit (KR).

7. The method according to claim 1 , wherein oxygen is again supplied to the cathode (K), before the electric voltage (U) of the fuel cell stack (BS) sinks below a lower voltage limit value.

8. The method according to claim 1 , wherein the electric voltage (U) of the fuel cell stack (BS) is regulated by a DC-DC transformer.

9. The method according to claim 1 , wherein the electric voltage (U) of the fuel cell stack (BS) is regulated by an adjustable electric load (KP).

10. The method according to claim 1 , wherein the hydrogen supply to the anode (A) is not interrupted during the changeover to the mode of reduced power.

11. The method according to claim 1 , wherein depleted cathode air (KA) is again returned to the cathode (K) via a cathode recirculation circuit (KR).

12. The method according to claim 1 , wherein oxygen is again supplied to the cathode (K), before the electric voltage (U) of the fuel cell stack (BS) sinks below a lower voltage limit value.

13. A method for operating a fuel cell system in a normal mode and a mode of reduced power output, wherein the fuel cell system comprises a fuel cell stack (BS) having at least one fuel cell (BZ) with an anode (A), a cathode (K), a proton exchange membrane, an anode fuel inlet, a cathode oxidant inlet, an anode outlet, a cathode outlet, a hydrogen supply and an oxidant supply, wherein the oxidant is air, the method comprising:

supplying hydrogen and air to the fuel cell to produce electrical energy,

interrupting the air supply to the cathode (K) side of the fuel cell during the changeover from the normal mode to the mode of reduced power output,

reducing an electric voltage (U) of the fuel cell stack (BS) by application of a current pulse sufficient to initiate an increased use of the oxygen in the cathode and rapidly decrease the electric voltage of the fuel cell stack to the mode of reduced power output, and

in the mode of reduced power output, regulating the electric voltage of the fuel cell stack by regulating an adjustable electric load so that the voltage is maintained below an upper voltage limit value and by supplying oxygen before exceeding the lower voltage limit value to maintain the electric voltage above a lower voltage limit value.