Activity-Drop of Hydrogen Evolution Reaction in LiNO 3 Based "Hydronium-in-Salt" Acidic Electrolytes on Platinum Enables Electrochemical Nitrate Reduction

The electrochemical nitrate reduction reaction (NO RR) involves multiple hydrogenation and deoxygenation steps, which compete with the hydrogen evolution reaction (HER). Therefore, NO 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 NO RR is significantly activated even in acidic lithium nitrate solutions at LiNO 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 NO RR while the suppression of HER-activity in the LiNO 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 LiNO solutions contrasts with that of a "water-in-salt" electrolyte in LiTFSI with the same concentration. The mechanism of NO RR activation in a HISE suggests facilitated proton-coupled electron transfers (PCETs) from H O to NO and subsequent reactive intermediates owing to the proximity between the two ions induced by the unique solvation structure blended with all ions together (Li + NO + H O ). In contrast, all the ions are separately hydrated at low concentrations of LiNO electrolytes. On the other hand, PCET from H O to NO in a high concentration LiNO electrolyte (e.g., 9 m) is not kinetically preferred, probably owing to the slow dissociation kinetics of H O, and therefore, H O reduction is not suppressed by the NO RR.