Hidden Assumption of Gouy-Chapman-Stern Model

Understanding the double layer at the electrode-electrolyte interface is a long-standing challenge in electrochemistry. The orthodox 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 herein that the serial connection tacitly prescribes a zero potential gradient at the solution-side boundary, which is, rigorously speaking, invalid. The bearing of this problematic assumption is pinpointed by comparing the double-layer impedance, which is analytically solved at the potential of zero charge, derived from the original and amended GCS models. Specifically, in the amended GCS model, the capacitance 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.