Progress and pitfalls in measuring the double-layer capacitance of platinum electrodes

Despite extensive research, the double-layer structure at Pt/aqueous electrolyte interfaces (quantified by the double-layer capacitance, C dl ) remains incompletely understood as even for the model Pt(111)/HClO 4 interface, anomalous C dl 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 C dl trends are not measurement artefacts, but instead reflect intrinsic properties of the Pt(111)/HClO 4 interface, necessitating continued investigation. We further highlight the complexity introduced by electrosorbed H ads and/or OH ads species resulting from catalytic H 2 O dissociation, which contribute an adsorption (pseudo-)capacitance, C ads . This complicates the deconvolution of C dl 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 C dl values from prior studies.