Ligand-engineered morphology and electronic structure modulation in iron coordination polymers for improved electrocatalytic ammonia synthesis

This study presents the design and synthesis of isostructural metal coordination polymers (MCPs) incorporating iron and hexaazatrinaphthalene ligands functionalized with different substituents (FeHATN-X, X = NO 2 , CN, Br). The ligand variations enable fine-tuning of geometric structures, pore environments, and electronic properties due to the polarity and intraframework interactions. Among them, FeHATN-NO 2 exhibits an enhanced electrocatalytic ammonia yield rate of 87 μg h −1 mg −1 with a Faradaic efficiency of 24 % at −0.4 V vs. RHE, outperforming its counterparts. Comprehensive characterizations reveal that the nitro substituent promotes smaller nanorod formation, increasing active site exposure and improving mass transport. Moreover, the establishment of Fe−O coordination sites can modulate the Fe center, facilitate stronger nitrogen adsorption and concurrently suppress the competing hydrogen evolution reaction, thereby collectively contributing to improved electrocatalytic activity. Furthermore, in-situ spectroscopic analysis confirms that iron coordination centers act as effective electron acceptors, facilitating the formation of *NH 2 intermediates at the Fe center. The system achieves peak efficiency at −0.4 V vs. RHE, where the conduction band potential promotes the involvement of free electrons in the reaction. This work underscores the potential of tailoring coordination environments and morphologies in MCPs to enhance electrocatalytic ammonia synthesis performance.