Measurement and interpretation of the double layer capacitance of Pt(111)/aqueous solution interfaces

The electric double layer (EDL) at Pt(111)/aqueous solution interfaces is, in some sense, a shelter for electrocatalysis; where one can find solace in exploring the fundamentals before venturing into the labyrinthine landscape of real-world intricacies. Although seemingly unpretentious, classical electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) play an indispensable role in quantitative investigation of this EDL. However, these measurements and interpretation are highly nontrivial, and three related issues are addressed in this Opinion article. First, we bring to light discrepancies among various measurements of the double layer capacitance Cdl, and recommend separating Cdl from possible pseudo-capacitances using EIS. Second, following our recommendation, we examine assumptions and limitations of the Frumkin–Melik–Gaikazyan (FMG) model that is the physical model used to extract Cdl from EIS data in the presence of chemisorption. Thirdly, we compare CV-derived Cdl and EIS-derived Cdl. We explain why EIS-derived Cdl is always positive even for the case where CV-derived Cdl assumes negative values due to partially charged adsorbates. We hope this Opinion can facilitate the advent of a converged understanding of the EDL at Pt(111)/aqueous solution interfaces.