Fuel cell stack end cell control methodology

A method of controlling the relative humidity in an electrochemical conversion device comprising:

• providing a fuel cell stack comprising a plurality of fuel cells stackingly arranged, at least one heater coupled to at least one end fuel cell disposed on at least one end of the fuel cell stack, wherein each fuel cell comprises a membrane electrode assembly;
• calculating a relative humidity for the fuel cell stack;
• selecting a relative humidity setpoint for the end cell membrane electrode assembly less than the relative humidity of the fuel cell stack;
• calculating an end cell membrane electrode assembly temperature setpoint using the relative humidity setpoint;
• calculating a heater temperature setpoint, the temperature being equal to the calculated end cell membrane electrode assembly temperature setpoint plus a computed temperature difference from the at least one end cell membrane electrode assembly to the heater;
• comparing the end cell heater temperature setpoint to a temperature reading of the end cell heater;
• and
• adjusting the temperature of the end cell heater until it reaches the heater temperature setpoint and thereby achieves the relative humidity setpoint of the at least end cell membrane electrode assembly.