The oxygen reduction reaction (ORR) is a crucial process in the fields of electrochemistry and hydrogen technology. Understanding the mechanism of this multistep reaction is essential for advancing hydrogen conversion devices, such as fuel cells and electrolyzers. Because it is a multistep electrochemical process, its mechanism can be investigated using electrochemical impedance spectroscopy (EIS), a technique employed to measure the characteristic times of individual steps. Despite the many advantages of EIS, the correct interpretation of Nyquist plots—which display the measurement results—remains challenging. In this study, a mathematical model was developed to simulate the electrochemical impedance spectra of the ORR based on an assumed reaction mechanism. Physical parameters that significantly affect the reaction pathway were identified, along with their influence on the characteristic times of individual steps and, consequently, on the EIS results. Using the developed model, the relationship between the EIS results and the ORR kinetic parameters, which arise from the intrinsic properties of the catalyst, was investigated. The influence of ORR parameters on measurability was also described by studying the percentage deviation of the results with respect to the measurement potential and the standard rate constants of each reaction step. Several findings regarding the theoretical physical limitations of the EIS technique for the ORR are presented. The insights obtained from the developed mathematical models could theoretically assist in interpreting EIS measurements on catalysts for proton-exchange membrane hydrogen fuel cells and water electrolysis.
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