Negative Resistance Values in Locally Resolved Impedance Spectra of Polymer Electrolyte Fuel Cells (PEFCs)

This work demonstrates that the appearance of negative resistance values in locally resolved impedance spectra of PEFCs is, in general, a direct consequence of down the channel inhomogeneities due to changing concentration of oxidant, fuel or water along the flow channels. It is shown that the local polarization resistance becomes negative, as the local change in i) concentration polarization and/or ii) ohmic loss exceeds the total perturbation voltage amplitude at low frequencies. This forces the local cell current to decrease despite a higher total cell polarization. For the first time, this effect is shown here as a consequence of fuel and water depletion along the flow channels of a linear PEFC. Based on our earlier experimental results for the effect of oxygen depletion in H-2/air PEFCs, we present a pseudo 2D transient model to further illustrate the impact of mass transport limitations along the cathode flow channels on locally resolved impedance response.