Tuning metal centers in hexaazatrinaphthalene tricarboxylic acid-based coordination polymers for efficient nitrogen electroreduction into ammonia

In this work, a series of metal-organic coordination polymers composed of transition metals and hexaazatrinaphthalene tricarboxylic acid ligands (M-HATNTA, M = Fe, Co, Cu) were prepared via a facile solvothermal method for efficient electrocatalytic nitrogen reduction to ammonia. Fe-HATNTA forms nanotubular structures with rough surfaces, whereas other polymers produce nanorods with smooth surfaces. Fe-HATNTA outperforms its counterparts, presenting a nitrogen fixation rate of 16.4 μg h −1 mg −1 with Faradaic efficiency of 12.3 % at −0.5 V vs RHE. The coordination of Fe can enhance conductivity by improving the accessibility of active centers through surrounding organic ligands and fostering strong electronic interactions between the organic ligands and metal nodes. Moreover, the one-dimensional nanotubular architecture enhances electron transfer along the axial direction. In-situ spectroscopic studies reveal that iron coordination sites, acting as electron acceptors, can promote the formation of the −NH intermediate and the optimal potential of −0.5 V vs RHE encourages the participation of free electrons in the electrocatalytic reaction, as it corresponds to the conduction potential of Fe-HATNTA. This study emphasizes the role of metal centers and group modifications in optimizing metal-organic coordination polymers for electrocatalytic ammonia synthesis.