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Ion-ion interactions in supercapacitor (SC) electrolytes are considered to have significant influence over the charging process and therefore the overall performance of the SC system. Current strategies used to weaken ionic interactions can enhance the power of SCs, but consequently, the energy density will decrease due to the increased distance between adjacent electrolyte ions at the electrode surface. Herein, we report on the simultaneous enhancement of the power and energy densities of a SC using an ionic mixture electrolyte with different types of ionic interactions. Two types of cations with stronger ionic interactions can be packed in a denser arrangement in mesopores to increase the capacitance, whereas only cations with weaker ionic interactions are allowed to enter micropores without sacrificing the power density. This unique selective charging behavior in different confined porous structure was investigated by solid-state nuclear magnetic resonance experiments and further confirmed theoretically by both density functional theory and molecular dynamics simulations. Our results offer a distinct insight into pairing ionic mixture electrolytes with materials with confined porous characteristics and further propose that it is possible to control the charging process resulting in comprehensive enhancements in SC performance.

Well-tailored mixtures of distinct ionic liquids can act as optimal electrolytes that extend the operating electrochemical window and improve charge storage density in supercapacitors. Here, we explore two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4). We study their electric double-layer behavior in the neat state and as binary mixtures on the external surfaces of onion-like carbon electrodes using quasielastic neutron scattering (QENS) and classical density functional theory techniques. Computational results reveal that a mixture with 4:1 EmimTFSI/EmimBF4 volume ratio displaces the larger [TFSI–] anions with smaller [BF4 –] ions, leading to an excess adsorption of [Emim+] cations near the electrode surface. These findings are corroborated by the manifestation of nonuniform ion diffusivity change, complementing the description of structural modifications with changing composition, from QENS measurements. Molecular-level understanding of ion packing near electrodes provides insight for design of ionic liquid formulations that enhance the performance of electrochemical energy storage devices.

Colloidal particles are mostly charged in an aqueous solution because of the protonation or deprotonation of ionizable groups on the surface. The surface charge density reflects a complex interplay of ion distributions within the electric double layer and the surface reaction equilibrium. In this work, we present a coarse-grained model to describe the charge regulation of various colloidal systems by an explicit consideration of the inhomogeneous ion distributions and surface reactions. With the primitive model for aqueous solutions and equilibrium constants for surface reactions as the inputs, the theoretical model is able to make quantitative predictions of the surface-charge densities and zeta potentials for diverse colloidal particles over a wide range of pH and ionic conditions. By accounting for the ionic size effects and electrostatic correlations, our model is applicable to systems with multivalent ions that exhibit charge inversion and provides a faithful description of the interfacial properties without evoking the empirical Stern capacitance or specific ion adsorptions.

The aim of this work is to develop a predictive model to describe the electrostatic behavior of 20 natural amino acids under diverse solution conditions. A coarse-grained model is proposed to account for the key ingredients of thermodynamic nonideality arising from the interaction of amino acids with various solvated ions in an aqueous solution including the molecular volume excluded effects, solvent-mediated electrostatic interactions, van der Waals attraction, and hydrophobic and hydration forces. With a small set of parameters characterizing the intermolecular interactions and dissociation equilibrium, the thermodynamic model is able to correlate extensive experimental data for the activity coefficients and solubility of amino acids in pure water and in aqueous sodium chloride solutions. Moreover, it predicts apparent equilibrium constants for the charge regulation of all amino acids in excellent agreement with experimental data. Importantly, the thermodynamic model allows for the extrapolation of the intrinsic equilibrium constants for amino-acids conversion among different charge states (viz., negative, neutral, and positive charges) thereby enabling the prediction of the charge behavior for all natural amino acids under arbitrary solution conditions.