Not Every Semicircle Is Interfacial: High-Frequency Shielding Artifacts in Three-Electrode Electrochemical Impedance Measurements

Abstract Electrochemical impedance spectroscopy (EIS) is widely used to investigate interfacial processes and transport properties of electrochemical systems. Three-electrode configurations are commonly employed to separate the impedance contributions from individual electrodes; however, the influence of reference electrode structures on high-frequency impedance measurements has received limited attention. In this work, we identify a high-frequency artifact in three-electrode EIS measurements caused by the insulating sheath of the conventional reference electrodes. Using Ag/Ag2O reference electrodes as a model system, an additional high-frequency semicircle was observed in the impedance spectra. Through systematic experiments, this feature is shown to be independent of the electrode materials, electrolyte composition, and instrumental parasitic capacitance. Finite-element simulations further demonstrate that the dielectric sheath around the reference electrode disturbs the local alternating electric-field distribution, resulting in an artificial relaxation process associated with dielectric shielding. The intensity of this artifact depends on the reference electrode geometry and electrolyte dielectric properties, and the same phenomenon is observed in both the aqueous and organic electrolytes. These results indicate that accurate three-electrode EIS measurements require not only sufficient ionic contact between the reference electrode and electrolyte but also minimal disturbance of the local electric-field distribution.