Nonmonotonic Charging, Overscreening, and Ion Competition in the Electric Double Layer of Platinum Quantified by X-ray Spectroscopy

Abstract The electric double layer (EDL) is an integral part of the local reaction environment in electrocatalysis. Indeed, the strong electric fields and modified interfacial water structure accompanying ion accumulation in the EDL have been shown to impact the performance of nearly every electrocatalytic reaction. However, quantifying interfacial ion accumulation experimentally is a long-standing challenge, leaving the atomic-level picture of electrolyte effects in electrocatalysis incomplete. Here, we address this challenge using an interface-sensitive in situ X-ray fluorescence spectroscopy approach based on a mesoporous Pt electrode to track interfacial ion concentration at the Pt-electrolyte interface in acidic media. We show that the EDL structure of the Pt electrode has a complex dependence on the applied potential and electrolyte composition. From a systematic analysis of ten electrolytes (including ions such as ClO4–, SO42–, K+, and Ca+), we show that adsorbates alter the surface free charge, resulting in nonmonotonic charging and overscreening. For nonadsorbing anions, we show that the concentration-dependent surface free charge follows classical Gouy–Chapman theory. At negatively charged surfaces, cations and protons compete to screen the surface free charge in the EDL, with a strong dependence on ion valency.