Activity-Drop of Hydrogen Evolution Reaction in LiNO₃ Based “Hydronium-in-Salt” Acidic Electrolytes on Platinum Enables Electrochemical Nitrate Reduction

The electrochemical nitrate reduction reaction (NO₃^-RR) involves multiple hydrogenation and deoxygenation steps, which compete with the hydrogen evolution reaction (HER). Therefore, NO₃^-RR driven in acidic media is challenging in spite of advantageous fast hydrogen transfers in its elementary steps. The findings presented in this article first demonstrate that the NO₃^-RR is significantly activated even in acidic lithium nitrate solutions at LiNO₃ concentrations exceeding 6 m on a Pt electrode (the highly effective catalyst for HER) by the formation of a "hydronium-in-salt" electrolyte (HISE), a new type of aqueous high concentration salt electrolyte. The observed enhancement of NO₃^-RR while the suppression of HER-activity in the LiNO₃ based HISE was verified by scanning electrochemical microscopy, electrochemical impedance spectroscopy, UV-vis/IR spectroscopy, and molecular dynamics simulations. The formation of a HISE in acidic LiNO₃ solutions contrasts with that of a "water-in-salt" electrolyte in LiTFSI with the same concentration. The mechanism of NO₃^-RR activation in a HISE suggests facilitated proton-coupled electron transfers (PCETs) from H₃O⁺ to NO₃^- and subsequent reactive intermediates owing to the proximity between the two ions induced by the unique solvation structure blended with all ions together (Li⁺ + NO₃^- + H₃O⁺). In contrast, all the ions are separately hydrated at low concentrations of LiNO₃ electrolytes. On the other hand, PCET from H₂O to NO₃^- in a high concentration LiNO₃ electrolyte (e.g., 9 m) is not kinetically preferred, probably owing to the slow dissociation kinetics of H₂O, and therefore, H₂O reduction is not suppressed by the NO₃^-RR.