Method for changing a fuel cell system over to a standby mode as well as such a fuel cell system

1. A method for changing a fuel cell system from a normal mode of operation over to a standby mode, the fuel cell system having a fuel cell stack comprising cathode spaces and anode spaces, an anode supply system, a cathode gas supply system as well as an external electric circuit, a maximum efficiency of the fuel cell system being exhibited at a first load, the first load defining an optimal system efficiency load, the method comprising the following steps:

a) reducing a load—via the electric circuit of the fuel cell stack—down to a load within a range from −1% to +5% of the optimal system efficiency load,

b) regulating or controlling an anode pressure down via the anode supply system so that an anode operating pressure is reduced,

c) in the meantime, maintaining and controlling a cathode gas feed via the cathode supply system in such a way that a pressure differential that sets in between the anode spaces and the cathode spaces does not exceed a prescribed maximum pressure differential,

d) switching off the cathode gas feed if a further pressure differential between the anode spaces and of the fuel cell stack and an environment has reached the prescribed maximum pressure differential, and

e) switching off the load via the external electric circuit at the latest when a prescribed minimum limit voltage of the fuel cell stack has been reached.

2. The method as recited in claim 1 wherein an electric energy of the fuel cell stack generated to switch off the power withdrawal from the stack in step (e) is fed to an electric consumer or to a battery connected to the fuel cell stack via the electric circuit.

3. The method as recited in claim 2 wherein the electric consumer is an auxiliary aggregate of the fuel cell system or an external consumer.

4. The method as recited in claim 1 wherein the switching off the cathode gas feed includes switching off a conveying device for a cathode operating gas.

5. The method as recited in claim 4 wherein the conveying device is a compressor.

6. The method as recited in claim 1 wherein a recirculation of the anode operating gas is maintained while the anode pressure is being controlled or regulated in step (b) and while the cathode gas feed is being maintained in step (c).

7. The method as recited in claim 1 wherein the load set in step (a) is within the range from −0.7% to +3% of the optimal system efficiency load.

8. The method as recited in claim 7 wherein the load set in step (a) is within the range from −0.5% to +1% of the optimal system efficiency load.

9. The method as recited in claim 1 wherein the prescribed maximum pressure differential is within a range from 0.1 bar to 0.3 bar.

10. The method as recited in claim 1 wherein the prescribed maximum pressure differential is within a range from 0.15 bar to 0.25 bar.

11. A fuel cell system with a fuel cell stack comprising:

cathode spaces and anode spaces;

an anode supply system;

a cathode gas supply system; and

an external electric circuit the fuel cell system performing the method as recited in claim 1 .

12. The method as recited in claim 1 wherein during the regulating or controlling the anode pressure down via the anode supply system, the anode operating pressure is reduced linearly.