In situ observation of the potential-dependent structure of an electrolyte/electrode interface by heterodyne-detected vibrational sum frequency generation

Abstract

Elucidating the structure of electrolyte/electrode interfaces is of essential importance not only for understanding of the fundamental process of electrochemistry but also for developing next-generation rechargeable batteries. In this study, we applied HD-VSFG spectroscopy to study a prototypical non-aqueous electrochemical interface of a platinum electrode in 0.1 M LiCF₃SO₃ acetonitrile (CH₃CN) solution, and measured Im χ⁽²⁾ spectra by changing the applied potential in the range of −0.8 V to 2.0 V. In the positive potential region, the positive bands assignable to acetonitrile appear in the CH₃ and CN stretch regions, and their positive signs indicate the CH₃-down orientation of acetonitrile at the interface. We also observed an SO₃⁻ stretch band of the anion of the electrolyte and found that the potential dependence of its intensity is similar to those of the CH₃ and CN bands of acetonitrile. These observations indicate that the CF₃SO₃⁻ anion is adsorbed at the platinum surface in the positive potentials, which induces CH₃-down orientation of acetonitrile at the interface. The present study demonstrates the advantages of HD-VSFG spectroscopy for studying electrochemical systems, and it opens a new way to investigate electrolyte/electrode interfaces at the molecular level.