System and method for controlling cathode stoichiometry to minimize RH excursions during transients

A method for controlling the cathode stoichiometry of fuel cell stack, said method comprising:

• defining different operating ranges of the current density generated by the fuel cell stack;
• identifying a desired cathode stoichiometry for each of the low power range, the mid-power range and the high power range;
• determining which range the current density generated by the stack is in;
• determining that the current density generated by the stack has changed from one range to another range for an extended period of time;
• and
• maintaining the cathode stoichiometry for the one range for a predetermined period of time after the current density generated by the stack has changed to the other range.

A fuel cell system comprising:

• a fuel cell stack generating a current density;
• a compressor for providing an airflow to the fuel cell stack;
• and
• a controller for controlling the compressor for providing a desirable cathode stoichiometry, said controller determining that the current density generated by the stack has changed from one current density range to another current density range, maintaining the cathode stoichiometry that is desirable for the one range for a predetermined period of time after the current density generated by the stack has changed to the other range, and then increasing or decreasing the cathode stoichiometry to a desirable value for the other range so as to compensate for any mismatch in thermal dynamics caused by the change in current density.

A fuel cell system comprising:

• a fuel cell stack generating a current density;
• a compressor for providing an airflow to the fuel cell stack;
• and
• a controller for controlling the compressor for providing a desirable cathode stoichiometry, wherein the controller defines a low power range, a mid-power range and a high power range of the current density generated by the fuel cell stack and identifies a desirable cathode stoichiometry for each of the low power range, the mid-power range and the high power range, said controller determining which range the current density generated by the stack is currently operating in, determining that the current density generated by the stack has changed from one range to another range, and maintaining the cathode stoichiometry of the one range for a predetermined period of time after the current density generated by the stack has changed to the other range.