Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Area of Science:

• Electrochemistry
• Analytical Chemistry
• Materials Science

Background:

• Understanding pH gradients is crucial for elucidating electrochemical reaction mechanisms.
• Separating pH effects from intrinsic electrolyte properties requires precise pH measurement.
• Existing methods often lack the resolution or specificity for dynamic diffusion layer analysis.

Purpose of the Study:

• To develop a novel pH sensor for in-situ measurement of pH gradients in electrochemical diffusion layers.
• To investigate pH changes during hydrogen evolution reaction (HER) on a gold electrode.
• To demonstrate the sensor's capability in a non-buffered electrolyte system.

Main Methods:

• Fabrication of a pH sensor by functionalizing a gold ultramicroelectrode with a 4-nitrothiophenol (4-NTP) self-assembled monolayer, converted to a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple.
• Utilizing a capacitive approach technique within a Scanning Electrochemical Microscope (SECM) setup for electrolyte-free and mediator-free tip approach.
• Measuring pH via cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential.

Main Results:

• The developed sensor successfully measured significant pH changes (up to three units) in the diffusion layer during HER in a non-buffered electrolyte (0.1 M Li2SO4), even at low current densities.
• The sensor captured the transient dynamics of the diffusion layer with high time resolution.
• Demonstrated high sensitivity, capable of measuring pH differences exceeding 8 units with a resolution better than 0.1 pH unit.

Conclusions:

• The novel electrochemical pH sensor provides a powerful tool for probing localized pH variations during electrochemical reactions.
• The sensor's high sensitivity and temporal resolution enable detailed studies of reaction mechanisms and electrolyte effects.
• This technology advances the understanding of interfacial phenomena in electrochemistry, particularly for reactions like hydrogen evolution.