Acidic Nitrate Electroreduction with Ultrahigh Energy Efficiency

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.