Operando Monitoring Interfacial pH Gradients and Ion Kinetics in Proton‐Coupled Electron Transfer Reactions through Dual‐Loop Electrochemical Methodology

Interfacial pH gradients significantly affect proton‐coupled electron transfer (PCET) reactions, yet quantifying these dynamics under operando conditions remains challenging. Here, we present a dual‐loop methodology that enables real‐time, high‐resolution mapping of interfacial pH fluctuations through probing hydrogen ion‐specific adsorption potentials (HISAP). We effectively decouple electric field effects from genuine changes in proton activity, facilitating direct assessment of pH behavior across various electrolytes. Our results reveal that alkaline media can exhibit pH deviations over one unit at 100 mA cm −2 , while buffered neutral systems display even greater gradients (ΔpH > 6) due to slow ion transport. By combining experimental data with Nernst‐Planck models, we find that ion‐specific mobility and buffer capacity are key to pH regulation. A 64% reduction in pH gradients occurs with increased buffer concentration, linking theoretical predictions with operando measurements. This reliable methodology spans the full pH range and high current densities, overcoming the limitations of traditional optical and probe‐based techniques and providing a universal platform for understanding interfacial ion dynamics and optimizing electrocatalyst performance in energy conversion applications.