Uncovering the catalyst/electrolyte interfacial process by frequency dispersion of capacitance

Electrochemical impedance spectroscopy (EIS) is a widely used technique to monitor the electrical properties of a catalyst under electrocatalytic conditions. Although it is extensively used for research in electrocatalysis, its effectiveness and power have not been fully harnessed to elucidate complex interfacial processes. Herein, we use the frequency dispersion parameter, n, which is extracted from EIS measurements (C_s = afⁿ⁺¹, −2 < n <  −1), to describe the dispersion characteristics of capacitance and interfacial properties of Co₃O₄ before the onset of oxygen evolution reaction (OER) in alkaline conditions. We first prove that the n-value is sensitive to the interfacial electronic changes associated with Co redox processes and surface reconstruction. The n-value decreases by increasing the specific/active surface area of the catalysts. We further modify the interfacial properties by changing different components, i.e., replacing the proton with deuterium, adding ethanol as a new oxidant, and changing the cation in the electrolyte. Intriguingly, the n-value can identify different influences on the interfacial process of proton transfer, the decrease and blocking of oxidized Co species, and the interfacial water structure. We demonstrate that the n-value extracted from EIS measurements is sensitive to the kinetic isotope effect, electrolyte cation, adsorbate surface coverage of oxidized Co species, and the interfacial water structure. Thus, it can be helpful to differentiate the multiple factors affecting the catalyst interface. These findings convey that the frequency dispersion of capacitance is a convenient and useful method to uncover the interfacial properties under electrocatalytic conditions, which helps to advance the understanding of the interface-activity relationship.