Interpreting full-frequency impedance spectrum for PEM electrolyzers: Distribution of relaxation times-based modeling

Among various hydrogen production technologies, proton exchange membrane water electrolyzers (PEMWEs) are promising thanks to their ability to operate at high and intermittent loads, high efficiency, and high hydrogen purity. The development and application of PEMWEs rely strongly on performance characterization and estimation techniques. Electrochemical impedance spectroscopy (EIS) is one of the most important non-invasive characterization tools for electrochemical devices such as PEMWEs. Nevertheless, modeling and interpreting the impedance spectrum remain an open challenge that hinders its application in PEMWEs. To bridge the gaps, a model-free distribution of relaxation times (DRT)-based approach is proposed to analyze EIS measured from in-operation PEMWEs. Moreover, the interpretation of the full frequency range including low-frequency inductive loops is investigated. To this end, experiments have been performed to measure the impedance spectra under different temperatures, cathode pressures, water flow rates, and current loads. Then, the DRT-based approach is applied to analyze the measured spectra. Conclusions have been drawn regarding the influence of various operating conditions on the performance of the PEMWE stack. Especially, the low-frequency inductive loops are systematically investigated for the first time to reveal their influencing factors and possible causes. The temperature is identified as the dominant influencing factor, followed by water flow rate and cathode pressure. This work provides useful insights into the PEMWE functionality through interpreting impedance spectra including low-frequency inductive loops and its application to PEMWEs.