Double-layer capacitance on Pt(111): Notes and hypotheses on experimental challenges

Platinum has not only singular activity for many reactions but also singular double-layer properties. Recent capacitance measurements at the Pt(111)-aqueous solution interface have shown deviations from what classical models of electrical double layer (EDL) describe. Origins of the deviations, which are fundamental to understanding the EDL effects in electrocatalysis, are not clear. Herein, using the Dolin-Ershler model that distinguishes double-layer capacitance and pseudocapacitance of specific adsorption, we compare three experimental methods, and critically assess the measurements in the literature. An alternative scheme of interpretation based on the hypothesis of specific adsorption of ions is examined. Implications and possible experimental validation of the hypothesis are also discussed.