Activity-Drop of Hydrogen\nEvolution Reaction in LiNO<sub>3</sub> Based “Hydronium-in-Salt”\nAcidic Electrolytes\non Platinum Enables Electrochemical Nitrate Reduction

The electrochemical nitrate reduction reaction (NO<sub>3</sub><sup>–</sup>RR) involves multiple hydrogenation and\ndeoxygenation\nsteps, which compete with the hydrogen evolution reaction (HER). Therefore,\nNO<sub>3</sub><sup>–</sup>RR driven in acidic media is challenging\nin spite of advantageous fast hydrogen transfers in its elementary\nsteps. The findings presented in this article first demonstrate that\nthe NO<sub>3</sub><sup>–</sup>RR is significantly activated\neven in acidic lithium nitrate solutions at LiNO<sub>3</sub> concentrations\nexceeding 6 m on a Pt electrode (the highly effective catalyst for\nHER) by the formation of a “hydronium-in-salt” electrolyte\n(HISE), a new type of aqueous high concentration salt electrolyte.\nThe observed enhancement of NO<sub>3</sub><sup>–</sup>RR while\nthe suppression of HER-activity in the LiNO<sub>3</sub> based HISE\nwas verified by scanning electrochemical microscopy, electrochemical\nimpedance spectroscopy, UV–vis/IR spectroscopy, and molecular\ndynamics simulations. The formation of a HISE in acidic LiNO<sub>3</sub> solutions contrasts with that of a “water-in-salt”\nelectrolyte in LiTFSI with the same concentration. The mechanism of\nNO<sub>3</sub><sup>–</sup>RR activation in a HISE suggests\nfacilitated proton-coupled electron transfers (PCETs) from H<sub>3</sub>O<sup>+</sup> to NO<sub>3</sub><sup>–</sup> and subsequent\nreactive intermediates owing to the proximity between the two ions\ninduced by the unique solvation structure blended with all ions together\n(Li<sup>+</sup> + NO<sub>3</sub><sup>–</sup> + H<sub>3</sub>O<sup>+</sup>). In contrast, all the ions are separately hydrated\nat low concentrations of LiNO<sub>3</sub> electrolytes. On the other\nhand, PCET from H<sub>2</sub>O to NO<sub>3</sub><sup>–</sup> in a high concentration LiNO<sub>3</sub> electrolyte (e.g., 9 m)\nis not kinetically preferred, probably owing to the slow dissociation\nkinetics of H<sub>2</sub>O, and therefore, H<sub>2</sub>O reduction\nis not suppressed by the NO<sub>3</sub><sup>–</sup>RR.