In-Situ Assessment of PEM Fuel Cells via AC Impedance at Operational Loads

PEM fuel cells in a Ballard Nexa stack were analyzed via AC impedance using a Gamry FC350 monitor and a TDI electronic load. Single MEAs, multiple MEA cell groups, and full 47 MEA stacks were examined. Impedance results were successfully fit to equivalent circuit diagrams containing charge transfer polarization resistances related to activation energies, uncompensated resistances related to the electrolytes, and double layer capacitances related to the surface area of the electrocatalysts. AC impedance was successfully conducted on the fuel cell stack at over 46 A stack currents (~383.3 mA/cm). From AC impedance data collected at different currents, the rate limiting behavior of various components and physical processes are determined. Good agreement is obtained between the circuit elements and physical processes determined via AC impedance and independent measurements of these same elements by other means. AC impedance appears to be a simple and easy method to implement in-situ real time diagnostic capability suitable for assessing the behavior and state-of-health of the PEM stack.