Square-Planar Tetranuclear Cluster-Based High-Symmetry Coordination Metal–Organic Polymers for Efficient Electrochemical Nitrate Reduction to Ammonia

Abstract

Metal–organic polymers (MOPs) are gaining booming attention as atomically precise single-site catalysts for electrochemical nitrate-to-ammonia conversion owing to their regular structures and tunable functionalities. However, a molecular-level understanding is still lacking for the design of more efficient MOP electrocatalysts. Here, we report the construction of high-symmetry coordination MOPs (Mn-TATB, Fe-TATB, and Co-TATB), utilizing square-planar tetranuclear building units [M₄(μ₄-O)(CO₂)₈] (M = Mn, Fe, or Co) bridged by 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H₃TATB) ligands. These MOPs possess distinct coordination motifs with well-defined porosity, high-density catalytic sites, accessible mass transfer channels, and nanoconfined chemical environments. Benefited from the unique metal–organic coordination framework, Co-TATB demonstrated a remarkable ammonia production Faradaic efficiency (FE_NH3) of ∼98% across a wide potential range (−0.7 to −1.0 V (vs RHE)) in the electrocatalytic nitrate reduction reaction (NITRR) and maintained stable performance over a long duration when tested in a flow cell at an industrially relevant current density of ∼332.1 mA cm^–2. Furthermore, in situ spectroscopic analyses, combined with theoretical calculations, elucidate the intrinsic reaction pathway of the Co-TATB model during the NITRR process. These findings offer insightful perspectives on the strategic design of electrocatalysts with symmetrical configurations for the purification of nitrate-containing wastewater and the green synthesis of ammonia.