Methods and devices for rejuvenating, supplementing, or bypassing an individual fuel cell or a group of fuel cells, in a fuel cell stack, is disclosed. The methods and devices provide for a mechanism for removing catalyst poisons on both the anode portion and the cathode portion of the fuel cell and providing an improvement in the operation of the electrolytes. A controller that controls a variable resistor or a variable power supply in parallel with an individual fuel cell or a group of fuel cells in a stack is utilized. Adjusting the resistive value of the variable resistor, or the voltage level of the variable power supply, directly controls the current in the fuel cell. In accordance with Kirchoff s Current Law, decreasing the fuel cell voltage increases the current through the fuel cell, while the fuel cell stack is operational, and poisons deposited on the anode and cathode electrocatalysts are removed, thereby rejuvenating the fuel cell. Typically, the removal of cell poisons only requires short, periodic pulsing of the fuel cell potential. Methods and devices for determining the limiting processes within a fuel cell thus enabling the optimization of the fuel cell stack performance to diagnose the fuel cell stack are also disclosed.