Addressing the Dynamic Nature of Uncompensated Resistance in Electrocatalyst Evaluation

Abstract Uncompensated resistance (Ru) directly affects every iR-corrected overpotential, polarization curve, Tafel slope, and stability metric used in electrocatalyst evaluation, yet it is often treated as a single constant measured before or after the experiment. This assumption fails for catalyst-modified and gas-evolving electrodes, where Ru varies with potential, current, time, catalyst state, wetting, degradation, and bubble coverage. Here we treat Ru as an operando state variable and introduce two state-resolved protocols that pair time- or potential-resolved Ru with the current and potential measured at the same electrode state. These protocols require only high-frequency impedance analysis and can be implemented on most electrochemical workstations without hardware modification. Time-resolved tracking separates non-Ohmic response changes from time-varying Ohmic loss during electrolysis: on Pt/C, it resolves Ru oscillations consistent with bubble growth and detachment and a slow non-Ohmic hydrogen evolution reaction drift hidden in the raw current; on RuO2, it captures a non-monotonic Ru trajectory consistent with activation followed by catalyst-layer degradation. Potential-resolved tracking assigns a potential-resolved Ru(E) to each point in a polarization curve. For RuO2, Ru(E) shows a mid-potential decrease associated with improved catalyst-layer conduction and a high-potential increase consistent with bubble-associated constriction of the ionic path. Because these contributions occur at different reaction stages and potential windows, no single Ru can correct the full curve. Fixed-Ru compensation consequently distorts apparent activity and local Tafel slopes, whereas state-resolved correction identifies the potential window least biased by Ohmic and bubble effects. These results establish Ru as a measurable dynamic descriptor for more reliable electrocatalyst benchmarking.