Despite extensive research, the double-layer structure at Pt/aqueous electrolyte interfaces (quantified by the double-layer capacitance, Cdl) remains incompletely understood as even for the model Pt(111)/HClO4 interface, anomalous Cdl trends have been reported. These trends were previously ascribed to differences in measurement techniques (i.e. dc methods such as cyclic voltammetry versus ac methods such as electrochemical impedance spectroscopy [EIS]). However, by repeating these measurements using EIS, we clarify that these anomalous Cdl trends are not measurement artefacts but instead reflect intrinsic properties of the Pt(111)/HClO4 interface, necessitating continued investigation. We further highlight the complexity introduced by electrosorbed Hads and/or OHads species resulting from catalytic H2O dissociation, which contribute an adsorption (pseudo)capacitance, Cads. This complicates the deconvolution of Cdl from total capacitance, a challenge further exacerbated by structure-dependent adsorption between different Pt facets. Our goal is to clarify how these factors affect capacitance interpretation at Pt/aqueous electrolyte interfaces, particularly highlighting the progress and challenges in accurately extracting Cdl values from prior studies.
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