Influence of electrode polarization on the capacitance of an electric double layer at and around zero surface charge

The influence of polarization of the electrode on the interfacial capacitance of a restricted primitive model planar double layer at and around zero surface charge is studied using a modified Poisson-Boltzmann theory for 1: 1 electrolytes. The polarization, which occurs due to a dielectric discontinuity at the electrode-electrolyte interface, is treated by imagining fictitious image charges within the electrode, which mimic surface polarization charges. Specifically, the cases (i) when the electrode is a metallic conductor with infinite relative permittivity and (ii) when the electrode is a low relative permittivity insulator are investigated. The capacitance around zero surface charge is seen to undergo a gradual transition from having a camel-shaped form with a minimum at low electrolyte concentrations to having a maximum at higher concentrations consistent with the trends observed in earlier works in the absence of surface polarization. However, the transition envelope shifts to the lower concentration side relative to the no polarization situation for case (i), while it shifts to the higher concentration side for case (ii). The overall capacitance behaviour for both the situations indicates that the effect of the image interactions can be substantial, especially at low temperatures.