Solvation Structure Modulation in Aqueous Electrolytes: Stabilizing Pyrazine Radicals and Facilitating Electrochemical Nitrate Reduction

Aqueous electrolytes are promising for use in various research and industrial fields owing to their sustainable and environmentally safe nature. However, their application is limited by the high reactivity of water, which constrains electrode reactions within a narrow electrochemical window. Here, we demonstrate how the solvation structure of highly concentrated aqueous electrolytes can be effectively modulated to enhance electrochemical performance in two distinct applications, as revealed by molecular dynamics (MD) simulations. First, the stabilization of protonated radical intermediates of pyrazine derivatives in water-in-salt electrolytes (WISEs) with 7–8 m LiTFSI is confirmed. Strong interactions between pyrazine derivatives and Li⁺-coordinated water, Li(H₂O)ₙ⁺, suppress molecular aggregation and protect radical intermediates from disproportionation and oxygen-induced degradation, while no similar solvation structure modification occurs with concentrated LiNO₃ or LiCl.1 Second, we show that the electrochemical nitrate reduction reaction (NO₃–RR) is significantly activated even in acidic LiNO₃ solutions at concentrations exceeding 6 m on Pt by the formation of a “hydronium-in-salt” electrolyte (HISE), accompanied by an activity-drop of the competing hydrogen evolution reaction.2 MD simulations verify that the unique mixed solvation structure blended with all ions together (Li⁺ + NO₃⁻ + H₃O⁺) induces proximity between NO₃⁻ and H₃O⁺, facilitating proton-coupled electron transfers to NO₃⁻ and subsequent intermediates, whereas ions are separately hydrated at low LiNO₃ concentrations. These results highlight solvation structure modulation as a general strategy to control radical stability and reaction selectivity in aqueous electrochemical systems.