Nonlinear analysis of a classical system: The double‐layer capacitor

The double‐layer capacitance (Cdl) of a heterogeneous interface dominates the high frequency band in impedance spectroscopy and is commonly handled as a pure capacitor in practical voltammetries. However, the results of nonlinear electrochemical impedance spectroscopy (NLEIS) performed on a classical redox couple system show significant harmonic peaks at the high frequency band. Numerical simulations based on the Helmholtz model and the Stern model were carried out to interpret experimental data. Good agreement between the nonlinear Stern model and the experimental data was found, implying that although Cdl is dominated by the linear Helmholtz capacitance, the nonlinear responses are inevitably introduced by the Gouy–Chapman capacitance and result in significant distortions to the linear approximation.

► We explore a redox couple system using nonlinear impedance spectroscopy. ► Experimental results show high amplitude harmonic currents at high frequency range. ► A linear capacitor does not generate harmonic currents at high frequency. ► Non-linearity from Gouy–Chapman–Stern theory yields the experimental response.