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

Abstract

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)/HClO₄ 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)/HClO₄ interface, necessitating continued investigation. We further highlight the complexity introduced by electrosorbed H_ads and/or OH_ads species resulting from catalytic H₂O 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.