The Poisson-Boltzmann (PB) equation remains the simplest and very efficient tool for describing distribution of charged particles near the macroscopic charged surfaces in many areas, such as biophysics, electrochemistry, chemical engineering, etc [1]. As is well known, the PB equation is based on the mean-field theory that makes its application to the real systems quite problematic. Firstly, the mean-field theory itself does not allow us to take into account the effects of the ionic correlations that are crucial for concentrated enough electrolyte solutions. Secondly, considering the solvent as a continuous dielectric medium makes it impossible to study the effects of the solvent molecular structure. These two reasons have motivated the researchers to improve the PB equation in the last two decades [2]. In present report a theoretical research [3,4] on the effects of the polarizable co-solvent in the electric double layer theory will be presented. The derivation of the modified PB equation with explicit account of the polarizable co-solvent molecules within both the field-theoretical formalism and density functional theory will be demonstrated. An influence of the co-solvent polarizability in combination with the excluded volume of the co-solvent molecules on the differential capacitance of electric double layer will be discussed.