Understanding impedance spectra of a PEM fuel cell via the distribution of transport times

In PEM fuel cells, oxygen is transported through three media: the channel, the gas diffusion layer, and the cathode catalyst layer. Transport in each medium has its own characteristic frequency, so three oxygen transport peaks are expected in the distribution of relaxation times (DRT) spectrum. However, a single transport peak has typically been demonstrated in the literature. In the high-frequency range, DRT spectra of PEMFCs exhibit multiple peaks of an unknown nature. We show that these inconsistencies and phantom peaks arise due to poor description of the oxygen and proton transport processes by the Debye kernel used in the DRT. Further, it is suggested to replace the Debye kernel with the composite kernel, which better describes the oxygen and proton transport processes. The Distribution of Transport Times (DTT) transformation is illustrated using two model and one experimental impedance spectra.