Electrochemical Capacitance of CO-terminated Pt(111) is Dominated by CO-Solvent Gap

TL;DR

This study combines experimental capacitance measurements with first-principles calculations to reveal that the dominant factor in the electrochemical capacitance of CO-terminated Pt(111) is the CO-solvent gap, not the molecule itself.

Contribution

It provides a detailed microscopic analysis showing the CO-solvent gap as the main contributor to capacitance, challenging previous assumptions about the role of the CO molecule.

Findings

Most of the interfacial potential difference occurs across the CO-solvent gap.

The CO-solvent gap reduces overall capacitance compared to bare platinum.

Capacitance is less affected by electrolyte concentration due to the gap.

Abstract

The distribution of electric fields within the electrochemical double layer depends on both the electrode and electrolyte in complex ways. These fields strongly influence chemical dynamics in the electrode-electrolyte interface, but cannot be measured directly with sub-molecular resolution. We report experimental capacitance measurements for aqueous interfaces of CO-terminated Pt(111). By comparing these measurements with first-principles density-functional theory (DFT) calculations, we infer microscopic field distributions and decompose contributions to the inverse capacitance from various spatial regions of the interface. We find that the CO is strongly electronically coupled to the Pt, and that most of the interfacial potential difference appears across the gap between the terminating O and water, and not across the CO molecule as previously hypothesized. This `gap capacitance' resulting from hydrophobic termination lowers the overall capacitance of the aqueous Pt-CO interface, and makes it less sensitive to electrolyte concentration compared to the bare metal.