Acidic Nitrate Electroreduction with Ultrahigh Energy Efficiency

Abstract Ammonia (NH 3 ) is an important feedstock for industry, an ideal energy carrier, and a perspective storage media for hydrogen. Recently, electrochemical nitrate (NO 3 −) reduction under acidic conditions has received considerable attention but it suffers from limited efficiency especially under low NO 3 − concentration. Here, we report an in situ formed positively charged polyethyleneimine‐modified Cu under acidic conditions as a catalyst‐electrolyte interface (CEI) for electrochemical NO 3 − reduction to NH 3 . Such CEI can effectively accumulate NO 3 − anions via static interactions and accelerate *NO hydrogenation to *NOH by weakening *NO intermediate adsorption on Cu site, thereby facilitating NO3−‐to‐NH 3 conversion. Such CEI delivers an increased NH 3 Faradaic efficiency (FE) of 83.5% and an impressive half‐cell energy efficiency (EE) of 37.1% in 10 mM NO 3 − solution (pH = 1). The NH 3 FE and EE can further increase to 90.2% and 44.1% in 0.5 M NO 3 −, respectively. The high EE of CEI surpasses previously reported catalyst performances for NO 3 − reduction. Finally, we demonstrate the feasibility of a novel NO 3 −‐furfural battery, showcasing a self‐power electrocatalytic system capable of simultaneously treating NO 3 − pollutants, generating value‐added NH 3 and upgrading biomass. This work offers valuable insights into the construction of a CEI to enhance the efficiency of NH 3 synthesis.