Combined DRT/HRA Diagnostics for PEM Electrolyzers and PEM Fuel Cell Systems

Abstract

Electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times (DRT) derived from it describe the dynamic response of an electrochemical system linearized around an operating point. Harmonic Ratio Analysis (HRA), in contrast, evaluates higher harmonics of a defined periodic current modulation and thereby accesses selected features of the nonlinear response. DRT indicates the relaxation-time ranges in which linear loss contributions change; HRA additionally indicates whether the nonlinearity, asymmetry, or saturation behavior of the response changes. The combined approach is particularly valuable when different fault conditions cause similar linear impedance changes or when local changes are not clearly visible in DC voltage values. Two technically relevant differential diagnoses are discussed in detail: CO poisoning versus dehydration and flooding in PEM fuel cells, and H₂ crossover versus two-phase and gasaccumulation conditions in PEM electrolyzers. In each case, the decisive factor is the joint evaluation of the DRT pattern, load-dependent HRA signature, persistence, recovery behavior, and independent process variables. A separate focus is placed on multi-channel measurements at the cell or cell-group level. Certain early or locally confined effects may not become clearly apparent in either the total stack voltage or conventional DC cell-voltage measurements above measurement resolution, natural scatter, and temporal filtering, even though the channel-resolved impedance and harmonic response may already have changed. Multi-channel DRT/HRA therefore adds particularly valuable spatial evidence to the temporal diagnosis. Following platform-specific validation, the method can be used as an electrical early-warning, localization, and differential-diagnostic approach; it does not replace direct gas, pressure, temperature or safety sensors.