Starvation, flooding, and dry‐out phenomena occur in polymer electrolyte membrane fuel cells (PEMFCs), due to heterogeneous local conditions, material inhomogeneity, and uneven flow distribution across the single cell active area and in between the individual cells. The impact of the load level and air feed conditions on the performance was identified for individual single cells within a 10‐cell stack. Analysis of the current density distribution across the active area at the cell level was correlated with electrochemical impedance spectroscopy to enable operando fault diagnostic without any impact of the applied analytical tools on the single cell behavior. Moreover, the combination of both technologies allows in‐depth analysis of fault mechanisms in fuel cell single cells with improved sensitivity. Current density distribution and the quantitative assessment of the performance homogeneity demonstrated high sensitivity to small humidity changes and allow the detection of critical events, such as dry‐out in single cells. Impedance analysis is more sensitive regarding polarization and diffusion limitations and allows detection of cell flooding. The combination of both techniques is required for reliable identification of air starvation faults.

A comprehensive description of impedance of electrochemical systems has been presented, along with the assumptions of the method used. In this method, a multisinusoidal current excitation signal is used. The changes of potential of both the electrodes and the potential difference between these electrodes are all registered simultaneously as a function of time. The proposed method offers the possibility of separately determining the instantaneous value of impedance of each electrode as well as the impedance of a two-electrode system. The short-time Fourier transform of time registers allow the determination of changes in the measured impedance values over time. The method has been successfully verified with an electrical equivalent circuit simulating a non-stationary two-electrode system. The values of the impedance of particular components during operation of the fuel cell under a changing load have been obtained.