Marc T.M. Koper: Electric Double Layer of Platinum Electrodes

Marc T.M. Koper (Leiden University): Electric Double Layer of Platinum Electrodes TACO Colloquium, 24 November 2025 Abstract: Platinum is the most studied electrode material in electrocatalysis, but its electric double-layer properties are still incompletely understood. Only Pt(111) has a true double-layer window, which charges purely electrostatically. However, the properties of the electric double layer of Pt(111) do not follow the classic Gouy-Chapman-Stern theory. Lower coordination sites and facets on platinum are always covered by either hydrogen or hydroxyl adsorbates, which have an important influence on its double-layer properties, as will be illustrated by recent results of double-layer measurements on stepped Pt single-crystal electrodes combined with ab initio molecular dynamics simulations and mean-field modeling. In addition, we show that at low electrolyte concentrations, which are often necessary for double-layer measurements, Pt(111) displays strong constant-phase element behavior, which complicates the accurate measurement of its double-layer capacitance. The possible origin of this nonideality will be discussed. Finally, the effect of adsorbates on the electric double layer of platinum will be discussed on the basis of hydrogen-covered Pt(111).