Anion Driven Electric Double Layer Structure and Capacitive Behavior in Organic Electrolytes for K-Ion Supercapacitors

Anion chemistry plays a critical role in determining interfacial structure and charge storage in electric double-layer capacitors, yet its microscopic influence remains unclear. Here, constant-potential molecular dynamics simulations are employed to examine anion-specific effects in potassium-ion supercapacitors using graphene electrodes and propylene carbonate-based organic electrolytes. By fixing the cation (K⁺) and solvent while varying the anion (TFSI⁻, PF6⁻, and ClO4⁻), we isolate the impact of anion geometry and ion-pairing behavior on electric double-layer formation. The results reveal pronounced anion-dependent differences in ion adsorption, interfacial structure, and capacitive response. The KTFSI electrolyte exhibits the highest integral capacitance, arising from enhanced accumulation of K⁺ ions at the electrode surface due to weakened ion-pair correlations. In contrast, PF6⁻ and ClO4⁻ form stronger ion pairs with K⁺, limiting cation availability and reducing capacitance. These findings demonstrate charge storage can be optimized by tunning the geometry and chemical form of anions in K-ion storage systems