Fast Locally Resolved Electrochemical Impedance Spectroscopy in Polymer Electrolyte Fuel Cells

A system for performing locally resolved electrochemical impedance spectroscopy in a segmented polymer electrolyte fuel cell (PEFC) has been developed. The impedance measurement is carried out for all segments in parallel. Due to this unique fast parallel approach, the measurement period for obtaining a locally resolved impedance spectrum is virtually independent of the number of segments used. The current density distribution and the corresponding locally resolved impedance spectra for a PEFC segmented along a serpentine cathode flow field and operated on pure H₂/O₂ and H₂/air are presented and discussed. For low humidity H₂/O₂ operation, the locally resolved impedance spectra clearly show that the performance loss toward the gas inlets can be attributed to drying effects. For H₂/air operation, the lower-frequency loops of the local impedance spectra increase in size as a function of position along the channels. The increase becomes more pronounced with decreasing air stoichiometry. This is likely to be due to cumulative flooding of the gas diffusion layer and a decreasing molar fraction of oxygen in the cathodic gas stream. The locally resolved impedance spectra show that large differences in local cell impedance can occur, i.e., the impedance spectrum of the entire cell is of limited value and can be misleading.