the electrochemical nitrate reduction reaction (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 MD 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+).