Distribution of relaxation times-based impedance analysis of incremental PEM water electrolysis cells

We present impedance spectra and related Distribution of Relaxation Times (DRT) analysis of pressurized PEM water electrolysis (PEMWE) single cells obtained with a newly developed test bench, including an incremental (zero-gradient) cell for homogeneous operating conditions. The setup enables reproducible electrochemical impedance spectroscopy (EIS) measurements over technically meaningful operating parameter ranges. DRT analysis reveals that current density variations (0.01-7 A cm-2) show significant issues with the common approach of attributing an electrochemical process to a fixed relaxation frequency. We demonstrate that individual peaks significantly shift their relaxation frequency from the mHz-range at low current densities (0.01 A cm-2) up to the kHz-range at 7 A cm-2. We show variations of current density, temperature, pressure, and water flow rate, which reveal five capacitive and one low-frequency inductive peaks. Our findings emphasize the importance of detailed operating parameter variations for accurately assigning and modeling electrochemical processes in PEMWE cells.