GouyChapmanStern

Gouy-Chapman-Stern is a foundational model in electrochemistry that describes the structure of the electric double layer at a charged interface between an electrode and an electrolyte. It combines two elements: a compact, near-surface Stern layer and a more distant diffuse layer described by Gouy-Chapman theory.

In the Gouy-Chapman component, the diffuse layer arises from the balance between electrostatic forces and thermal motion of ions in the solution. Ions rearrange themselves so that the potential decays from the electrode into the bulk solution, a distribution governed by the Poisson-Boltzmann equation. The resulting diffuse layer contributes a potential-dependent capacitance that decreases with distance from the surface and depends on the ionic strength of the electrolyte.

The Stern part introduces a compact layer of ions that are specifically adsorbed or strongly solvated at or very near the electrode surface. This layer behaves approximately as a parallel-plate capacitor with a nearly constant capacitance over small potential ranges, effectively separating immersion effects from diffuse-layer behavior.

When combined, the Gouy-Chapman-Stern model treats the total double-layer capacitance as a series combination of the Stern capacitance and the diffuse-layer capacitance: 1/C_dl = 1/C_S + 1/C_GC. This provides a more accurate description of observed capacitance versus potential and concentration than either component alone, and it is widely used to interpret electrochemical impedance, cyclic voltammetry, and related measurements.

Limitations include its mean-field nature and neglect of specific adsorption beyond the Stern layer, finite ion size, and correlations. Extensions incorporate modified Poisson-Boltzmann approaches and non-ideal effects to better reflect real interfaces.