The impurity effect on the capacitance of ionic liquid-based supercapacitors: A classical density functional theory study of ionic liquids inside the spherical cavity of the porous electrode

Abstract

The effect of impurities on the capacitance curve has been investigated for ionic liquid-based supercapacitors. The coarse-grained model in the framework of classical density functional theory (CDFT) was used to predict the electric double layer (EDL) formed at the concave wall of the spherical cavity of a porous electrode. Our results demonstrate that the adsorption and desorption of impurities by the cavity have a significant impact on the shape of the capacitance curve. In some cases, a bell-camel shape transition in the capacitance curve occurs due to the adsorption of impurities. By increasing the impurity concentration and its attraction with the cavity, the position of the observed maximums in the camel shape of the capacitance curves shifts toward a higher electrode potential. In addition, we have shown that impurity adsorption affects the value of the zeta potential by reducing the total net ion charges stored inside the cavity. Increasing the electrode potential to a specific value causes the total desorption of impurities; hereinafter, we call that the potential of zero impurity adsorption, ΨZIA. The amount of this potential depends on the concentration of impurities, cavity size, impurity-cavity attraction, and the shape of ions in ionic liquids. As we expected, the impurity has no effect on the amount of capacitance for electrode potentials higher than ΨZIA. Finally, the differential capacitance curves for symmetric impure ionic liquid in spherical cavities have been compared to those for slit pores. Results show the same behavior for both of them, but the effect of impurity can be ignored at lower electrode potential in the case of spherical cavities due to the more confinement effect.