Charge storage in water-in-salt electrolytes: Insights from alkali metal trifluoroacetates

• Cation-dependent solvation governs spontaneous ion adsorption in microporous carbons • Na⁺, K⁺, and Cs⁺ salts exhibit distinct concentration-dependent pore accessibility and wettability • Ion pairing and aggregation modulate hydration structure and electrosorption behavior • CsTFA delivers superior charge storage due to enhanced confinement and ionophilicity The performance of energy storage devices is strongly dependent on electrolyte properties and their interaction with porous carbon electrodes. This report presents a detailed discussion of the use of trifluoroacetate (TFA)-based water-in-salt (WIS) electrolytes for supercapacitors. Raman spectroscopy is used to analyse contrasting interactions between group 1 cations and TFA. Nuclear Magnetic Resonance (NMR) spectroscopy further confirms differences in ion confinement, showing that CsTFA exhibited enhanced ion adsorption within carbon nanopores, correlating with its superior charge storage capacitance. Additionally, analysis of NMR data revealed that KTFA displayed lower ion adsorption within micropores compared to NaTFA and CsTFA, suggesting lower ionophilicity and explaining the lower capacitance of the K + salt. These differences are attributed to contrasting solvation behavior of Na⁺ and Cs⁺, where Na⁺ strongly interacts with the carbonyl group of the acetate at lower concentrations, but transitions to direct ion pairing at higher concentrations, whereas Cs⁺ favors ion clustering and weaker coordination across all concentrations. Potential of zero charge (PZC) measurements reinforce these findings, revealing shifts in ion adsorption behavior with increasing electrolyte concentration. Electrochemical analysis demonstrated that 20 m CsTFA achieved the highest capacitance while 10 m CsTFA exhibited the fastest charge transfer and highest energy and power density. Adsorption properties of TFA salts on carbon surfaces are of direct relevant to current fluoro-chemical remediation strategies.