Online resistance diagnostics based on a time-domain step-response analysis for stack-level proton exchange membrane water electrolyzers

Proton exchange membrane (PEM) water electrolyzers require reliable resistance monitoring to assess performance and durability at the stack level. However, electrochemical impedance spectroscopy is not readily applicable in practical systems owing to measurement time, operational constraints, and hardware requirements. This study aims to develop and validate a time-domain step-response analysis (SRA) framework for estimating stack-level resistance in PEM water electrolyzers, using transient voltage response to small current steps. The framework enables separation of key resistance components from transient responses without reliance on frequency-domain modeling. Compared with conventional time-domain techniques, including current interruption and switching methods, the proposed framework allows resistance estimation without interrupting operation or requiring additional switching hardware, thereby supporting in situ stack-level applications. The framework was validated experimentally using single-cell and three-cell stack configurations under representative operating conditions. Under optimized excitation, resistance values derived from the step response deviated by approximately 2–3% from galvanostatic impedance results, while measurement time was reduced from several minutes to approximately 1 s. Systematic evaluation across current densities of 0.25–1.0 A cm−2 and pulse durations of 0.1–3 s defines the applicable stack-level operating range in which estimation deviation remained within ±5%, thereby indicating the applicability of the method under practical operating conditions. These findings demonstrate that SRA enables rapid, minimally intrusive resistance estimation under practical operation conditions and supports real-time, stack-level diagnostics in PEM electrolyzer systems.