Dual Zn5‐NiS4 Sites in a Redox‐Active Metal‐Organic Framework Enables Efficient Cascade Catalysis for Nitrate‐to‐ammonia Conversion

Electrocatalytic Nitrate Reduction to Ammonia (eNO3RR) offers a promising solution to both environmental pollution and the sustainable energy conversion. Here we propose an efficient cascade catalytic mechanism based on a dual Zn5‐NiS4 sites, orderly assembled in a redox‐active metal‐organic framework structure, which separately promotes the reaction kinetics of nitrate‐to‐nitrite and nitrite‐to‐ammonia conversions. Specifically, the Zn5 clusters adsorb and selectively reduce the NO3− to NO2−, whereas [NiS4] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO2− to produce NH3. To this end, the bimetallic Zn5‐NiS4TP MOF was synthesized based on the redox‐active ligand [Ni(C2S2(TPCOOH)2)2]. A maximum ammonia production rate of 23650.63 µg·h−1·mg−1site and faradaic efficiency 92.57% was achived by Zn5‐NiS4TP MOF under neutral conditions. To validate the critical role of dual Zn5‐NiS4 sites, Mn5‐NiS4TP and Cd2‐NiS4TP were synthesized as control samples, together with Zn‐TTFTB, Zn‐NiS4Ph and other Zn5‐cluster‐based MOFs applied for the investigation of electrocatalytic nitrate reduction. Our results indicated that substitution by ‐thienyl instead of ‐phenyl group increases the S‐heteroatom content, improves the conductivity and facilitates electron transfer. Furthermore, Density Functional Theory (DFT) calculations of the energy changes for the reduction of each species could rationalize experimental results.