Double-layer structure of the Pt(111)-aqueous electrolyte interface

We present detailed measurements of the double-layer capacitance of the Pt(111)-electrolyte interface close to the potential of zero charge (PZC) in the presence of several different electrolytes consisting of anions and cations that are considered to be nonspecifically adsorbed. For low electrolyte concentrations, we show strong deviations from traditional Gouy-Chapman-Stern (GCS) behavior that appear to be independent of the nature of the electrolyte ions. Focusing on the capacitance further away from PZC and the trends for increasing ion concentration, we observe ion-specific capacitance effects that appear to be related to the size or hydration strength of the ions. We formulate a model for the structure of the electric double layer of the Pt(111)-electrolyte interface that goes significantly beyond the GCS theory. By combining two existing models, namely, one capturing the water reorganization on Pt close to the PZC and one accounting for an attractive ion-surface interaction not included in the GCS model, we can reproduce and interpret the main features the experimental capacitance of the Pt(111)electrolyte interface. The model suggests a picture of the double layer with an increased ion concentration close to the interface as a consequence of a weak attractive ion-surface interaction, and a changing polarizability of the Pt(111)-water interface due to the potential-dependent water adsorption and orientation.

Automated summary

This study presents detailed measurements of the double-layer capacitance at the Pt(111)-electrolyte interface near the potential of zero charge (PZC). Strong deviations from traditional Gouy-Chapman-Stern behavior are observed at low electrolyte concentrations, independent of the nature of the electrolyte ions. The capacitance further away from PZC and the trends for increasing ion concentration show ion-specific effects related to ion size or hydration strength. A model is formulated to explain the structure of the electric double layer at the Pt(111)-electrolyte interface, going beyond the Gouy-Chapman-Stern theory.