Numerical Analysis of Electric Double Layer Capacitors with Mesoporous Electrodes: Effects of Electrode and Electrolyte Properties

A theoretical study is presented of the behavior of electric double layer (EDL) supercapacitors constructed from mesoporous activated carbon electrodes. A threedimensional (3D) computational model is developed to predict the equilibrium charge distribution within the supercapacitor as a function of key electrode and electrolyte properties. The model is based on the modified Poisson− Boltzmann (MPB) equation and takes into account the impact of critical device parameters, including the size of the ions, the bulk ion concentration, the field-dependent permittivity of the electrolyte, the specific surface area of the electrodes, and the applied voltage. A key feature of the MPB model is that it limits the accumulation of ionic charge at the electrode− electrolyte interface by accounting for the finite size of the ions. This provides more accurate predictions of capacitance than models based on a point charge approximation. The model is used to perform a systematic parametric analysis of device performance and quantify the relative impact of various parameters on the gravimetric capacitance. It is also used to compare the capacitance obtained using organic versus inorganic electrolytes. The analysis confirms that aqueous electrolytes are able to attain a higher capacitance and power density. The model can be adapted to analyze the effects of arbitrary electrode morphologies and a broad range of electrolyte properties. It provides unique insight into the internal physics of an electrochemical cell and is well suited for the rational design of novel EDL supercapacitors.