Electrolyte Cation Effects on Interfacial Acidity and Electric Fields

Area of Science:

• Electrochemistry
• Surface Science
• Spectroscopy

Background:

• Electrolyte cations influence electrocatalysis by affecting interfacial electric fields, structure, and local pH.
• The precise roles of different electrolyte cations remain unclear, necessitating further investigation.

Purpose of the Study:

• To investigate the effect of electrolyte cation identity on interfacial acidity and electric fields.
• To understand how applied potential modulates these interfacial properties in mixed self-assembled monolayers (SAMs).

Main Methods:

• Utilized surface-enhanced infrared absorption spectroscopy (SEIRAS) on mixed SAMs of 4-mercaptobenzoic acid and 4-mercaptobenzonitrile on gold.
• Employed various electrolytes including Li+, Na+, K+, ND4+, and tetrabutylammonium (TBA+) phosphate.

Main Results:

• Small cations (Li+, Na+, K+, ND4+) showed minimal effects on interfacial acidity and electric fields.
• The large TBA+ cation resulted in significantly higher electric fields, less sensitive to applied potential, and a more basic SAM interface.
• Combined cation identity and applied potential shifted interfacial acid/base equilibria by over three orders of magnitude.

Conclusions:

• Electrolyte cation identity profoundly impacts interfacial properties, including electric field strength and acidity.
• The size and approach of cations to the surface dictate electric screening and potential decay within SAMs.
• These findings offer fundamental insights into cation effects on interfacial structure and proton transfer for electrochemical applications.