Hydration Asymmetry and Solvent-Mediated Charge Balancing in Microporous Carbons: An NMR Study of Aqueous and Organic Electrosorption

Abstract

Aqueous electrolytes are attracting increasing interest in supercapacitor applications due to their high sustainability and ionic conductivity relative to conventional organic systems. However, the distinct solvation properties of water and their impact on (electro)sorption mechanisms remain poorly understood, particularly the role of hydronium (H3O+) and hydroxide (OH–) ions in charge storage. In this work, we utilize in situ and MAS NMR spectroscopy to investigate a series of aqueous and organic trifluoromethanesulfonate (triflate, OTf–) electrolytes within commercial activated carbon. We reveal a fundamental link between ion solvation energetics, water dissociation, and in-pore charge balancing. Our results demonstrate that an increase in the in-pore ion populations does not necessarily lead to higher capacitance; instead, improved performance is correlated with more acidic in-pore environments. Furthermore, we show that hydration asymmetry between charge-dense cations and charge-diffuse anions dictates the degree of water dissociation, providing a chemical handle to promote acidic environments and enhance capacitance. Finally, in situ NMR measurements elucidate the charging mechanisms of these systems, highlighting how solvent choice fundamentally alters the electrosorption process and overall electrochemical performance.