Application of the Kramers Kronig relations to locally resolved impedance data of polymer electrolyte fuel cells

The Kramers Kronig (KK) relations are applied to locally resolved impedance data as obtained from a simple, analytic ‘down the channel’ impedance model of a polymer electrolyte fuel cell (PEFC) air cathode. It is shown that the transport and superposition of oxygen concentration oscillations in gas flow direction can lead to non-causal local cell impedance spectra, while the calculated local cell admittance always fulfills the KK relations. This is explained by the homogeneous distribution of the ac perturbation voltage over the electrode area due to the high electrical conductivity of the bipolar plates. Hence, the KK relations must be applied to the locally resolved admittance data of PEFCs to check if the measured spectra can be a physical response of the system.