Acidic Nitrate Electroreduction with Ultrahigh Energy Efficiency

Ammonia (NH3) is an important feedstock for industry, an ideal energy carrier, and a perspective storage media for hydrogen. Recently, electrochemical NO3− reduction under acidic conditions has received considerable attention but it suffers from limited efficiency especially under low NO3− concentration. Here, we report an in‐situ formed positively charged polyethyleneimine‐modified Cu under acidic conditions as a catalyst‐electrolyte interface (CEI) for electrochemical NO3− reduction to NH3. Such CEI can effectively accumulate NO3− anions via static interactions and accelerate *NO hydrogenation to *NOH by weakening *NO intermediate adsorption on Cu site, thereby facilitating NO3−‐to‐NH3 conversion. Such CEI delivers an increased NH3 Faradaic efficiency (FE) of 83.5% and an impressive half‐cell energy efficiency (EE) of 37.1% in 10 mM NO3− solution (pH = 1). The NH3 FE and EE can further increase to 90.2% and 44.1% in 0.5 M NO3−, respectively. The high EE of CEI surpasses previously reported catalyst performances for NO3− reduction. Finally, we demonstrate the feasibility of a novel NO3−‐furfural battery, showcasing a self‐power electrocatalytic system capable of simultaneously treating NO3− pollutants, generating value‐added NH3 and upgrading biomass. This work offers valuable insights into the construction of a catalyst‐electrolyte interface to enhance the efficiency of NH3 synthesis.