Negative Capacitance of an Electrolytic Cell in the Absence of Bias Potential

We extend the Poisson–Nernst–Planck model to the case of an electrolytic cell limited by ohmic electrodes that are partially polarizable with different affinities for anions and cations. We show that the finite conductivity of the electrodes in combination with the different ionic diffusion coefficients for anions and cations may give rise to negative capacitance behavior of the cell impedance in the absence of any bias. The range where the negative capacitance character of the electrolytic cell appears is localized. When ambipolar diffusion and electrode polarization effects are simultaneously present, the real part of the electrical impedance shows up to three plateaux.