Defects-Rich Mn2V2O7/Fe3S4 Catalysts for Ammonia Synthesis via a Bifunctional N2-Glycerol Electrolysis Cell: Applying the Concept of Solid-State Lewis Acid/Base as Catalysis-Active Sites

Abstract This study presents an innovative ternary non-noble metal oxide/sulfide composite electrocatalyst, (Mn,V)4O7−x/Fe3S4−y, designed to enhance the efficiency and selectivity of the ambient-condition electrochemical nitrogen reduction reaction (N2RR). Synthesized on nickel foam via a hydrothermal method, this multidefective, bifunctional catalyst aims to replace the energy-intensive Haber Bosch process for ammonia (NH3) production. By incorporating manganese into an iron-vanadium system, the ternary catalyst achieves a remarkable NH3 yield rate of 5.041 mg cm−2 h−1 and a Faradaic efficiency (FE) of 31.7% at −0.7 V vs RHE in 0.5 M Na2SO4 electrolyte. This performance roughly doubles that of the binary V2O5/FeS2 baseline catalysts. In a single-stack N2RR||OER cell at 2.5 V, the system attained an NH3 yield of 3.505 mg cm−2 h−1, 45% FE, and 56.2% energy efficiency, consuming 29.1 kWh KgNH3−1. To further reduce electricity demands down to 16.1 kWh KgNH3−1, the conventional anodic oxygen evolution reaction was replaced with a glycerol oxidation reaction (N2RR||GOR). The catalyst's outstanding yield, selectivity, and overall stability stem from non-stoichiometry-derived multivalent defects acting as catalytically active centers. Specifically, metallic Lewis acids and nonmetallic Lewis bases work collaboratively to strongly trap, adsorb, and activate inert N2 molecules. Ultimately, this rational, defect-engineered catalyst design represents a highly significant, industrially viable advancement for ambient nitrogen fixation.