Solvent-controlled ion association and EDL stability in potassium acetate electrolytes for carbon-based supercapacitors

Solvent properties strongly influence ion transport, interfacial charge storage, and long-term durability in supercapacitors. Here, 2 mol kg −1 potassium acetate (KAc) in water and methanol is compared to clarify how solvent-dependent solvation governs electric double-layer (EDL) formation and stability in nanoporous carbon electrodes. Molecular dynamics simulations reveal pronounced differences in ion diffusion, ionicity, coordination environment, and ion–ion interaction energetics in the two electrolytes, while Raman spectroscopy confirms stronger ionic association in methanolic KAc. Although the aqueous electrolyte exhibits faster ion transport, lower resistance, and higher capacitance, the methanolic electrolyte forms a more stable interfacial environment. We hypothesize this stability arises because strong ion pairing and large desolvation barriers kinetically hinder parasitic side reactions, thereby preserving predominantly EDL-based charge storage at high cell voltages. Electrochemical analysis of symmetric carbon/carbon supercapacitors further reveals distinct solvent-dependent electrode potential distributions, charge-storage asymmetry, and floating stability. At 2.0 V, methanol-based capacitors retain 65% energy efficiency after 100 h with substantially lower leakage losses, whereas the aqueous system retains only 13%. These findings demonstrate that tuning solvation structure through solvent selection provides an effective route to balancing ion transport, voltage tolerance, and long-term stability in carbon-based supercapacitors.