Impurity-Size Effects on the Potential of Zero Charge, Capacitance Behavior, and Interfacial Phenomena in Ionic Liquid-Based Supercapacitors Using Classical Density Functional Theory

The effect of impurity size on the electrochemical behavior of ionic liquid-based supercapacitors has been investigated. A coarse-grained classical density functional theory (CDFT) model was used to study an asymmetric ionic liquid confined inside the spherical cavity of a porous electrode. The electrolyte consists of dimeric cations, monomeric anions, and neutral impurity molecules with different sizes. Our results show that smaller impurities are adsorbed in greater amounts and remain inside the cavity over a wider range of electrode potentials. As the impurity size increases, impurity adsorption decreases, the potential of zero charge (PZC) shifts toward more negative values, and counterion selectivity generally increases. At an electrode potential of −0.05 V, surface charge amplification (SCA) is observed for impurity-size ratios of α = 1.0, 1.3, and 1.66, whereas charge reversal (CR) occurs for α = 2.0, where α denotes the impurity-to-anion size ratio. The presence of impurities changes the capacitance curve from bell-shaped for the pure ionic liquid to camel-shaped for the impure ones. At sufficiently high electrode potentials, complete impurity desorption causes the capacitance curves to become nearly independent of impurity size. Finally, the Stern-region thickness is influenced by impurity adsorption, with sharper variations for larger impurities and more gradual changes for smaller ones.