Ionic liquid–electrode interface: From one law to fit them all to one model to predict them all

The electric double layer (EDL) – arguably the central concept in electrochemistry – remains the nut everyone wants to crack to tune its properties and, thus, control the performance of electrochemical devices. One such property is the differential capacitance that determines the energy density in supercapacitors. In this communication, we show that the capacitance–potential dependence can be fitted with a four-parameter model for ionic liquid–electrode interfaces, where steric packing naturally results in a power-law scaling. First, we demonstrate how these parameters can be evaluated and used to interpret the EDL structure and properties. Second, we demonstrate how the model enables predictions of both differential capacitance and energy density. • The presented four-parameter model fits computed and measured EDL data. • The model links micro- and macroscopic properties. • The model extracts physical parameters from simulations and experiments. • The model outperforms the mean-field theory at high absolute potentials. • The model predicts the differential capacitance and energy density.