Preprint of "Beyond Steady-State Limitations of Electrochemical Impedance Spectroscopy: Time-Resolved Decoupling of Transient Processes"

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for separating individual processes in electrochemical systems. However, conventional EIS requires the system to remain stationary during measurement, thereby limiting its applicability to transient phenomena. In this study, we present a novel approach for obtaining impedance spectra with high temporal resolution during rapidly evolving electrochemical processes. Instead of measuring a complete spectrum in a single experiment, individual isofrequency responses were recorded during repeated transient events, each initiated from the same state. The interpolation of these responses enables the reconstruction of instantaneous impedance spectra, which can be further analyzed using standard EIS methods, including the distribution of relaxation times and equivalent circuit fitting, while providing direct access to the temporal evolution of individual electrochemical processes. The method was demonstrated on proton and anion exchange membrane water electrolyzers, where instantaneous impedance spectra were successfully reconstructed with a sub-second temporal resolution. This approach extends impedance spectroscopy beyond steady-state operations and provides a powerful tool for investigating transient phenomena in electrochemical energy conversion and energy storage devices.