Structure of the Electrical Double Layer Revisited: Electrode Capacitance in Aqueous Solutions

Area of Science:

• Electrochemistry
• Physical Chemistry
• Surface Science

Background:

• The electrical double layer (EDL) structure at electrode-electrolyte interfaces is crucial for electrochemical processes.
• Existing theories, like the Gouy-Chapman-Stern model, predict concentration-dependent EDL capacitance.
• Understanding EDL capacitance is vital for applications in sensors, batteries, and corrosion prevention.

Purpose of the Study:

• To investigate the electrical double layer structure at planar electrode-aqueous solution interfaces.
• To determine the dominant capacitance component (Stern vs. diffuse layer) across varying ionic concentrations.
• To experimentally validate or challenge predictions of the Gouy-Chapman-Stern theory.

Main Methods:

• Utilized Electrical Impedance Spectroscopy (EIS) to measure impedance of NaCl and surfactant solutions.
• Analyzed admittance spectra to directly infer electrode capacitance.
• Employed equivalent circuit regression to further determine capacitance values.

Main Results:

• Electrode capacitance remained relatively constant across a wide range of investigated concentrations.
• Observed capacitance values were inconsistent with Gouy-Chapman-Stern predictions at low concentrations.
• The Stern layer capacitance was found to consistently dominate the overall electrode capacitance.

Conclusions:

• The Stern layer capacitance is the primary determinant of electrode capacitance, irrespective of bulk solution concentration.
• Diffuse layer capacitance becomes significant only at ionic strengths approaching 1 mM.
• Experimental findings necessitate a refinement of EDL models, particularly concerning low ionic strength conditions.