A Hidden Assumption of Gouy-Chapman-Stern Model

Understanding the double layer at the electrode-electrolyte interface is of fundamental importance to electrochemistry, and also lays the basis for rational design of energy storage and conversion technologies. The prevailing Gouy-Chapman-Stern (GCS) model and its many derivatives invariably picture the double layer as a serial connection of a compact layer and a diffuse layer. We unravel that these models based on the serial connection tacitly prescribe a zero potential gradient at the solution-side boundary. This assumption is generally invalid, resulting in an incorrect expression for the double-layer impedance. Amendment of this deficiency gives out a revised analytical expression for the double-layer impedance at the potential of zero charge with new features. Specifically, the contribution of the compact layer now shows frequency dispersion. The deviation between the original and amended models is greater when the double layer is confined in narrower space. This work changes our basic understanding of double layer model and its impedance response.