Electrocatalytic nitrate-to-ammonia conversion using Mo-S-Fe₃O₄ nanoplates on foam iron electrode: Synergistic effects of Lewis-acid and Lewis-base sites

The NO₃⁻ electroreduction process presents a promising alternative to the Haber-Bosch method by offering dual benefits: removal of nitrates from wastewater coupled with efficient NH₃ production. Herein, Mo and S doped Fe₃O₄ nanoplates grown on foam Fe (Mo-S-Fe₃O₄@FF) with electron-rich Mo^δ+ state was synthesized through a facile hydrothermal approach. Notably, Mo-S-Fe₃O₄@FF electrode achieved a nearly fourfold enhancement in ammonium yield compared to foam Fe electrode, while also enhancing the NH₃ Faradaic efficiency. At − 1.3 V vs. Ag/AgCl in a K₂SO₄ + KNO₃ solution, the Mo-S-Fe₃O₄@FF electrode attained a NH₃ Faradaic efficiency of 91.3 %. Furthermore, performance was observed to improve with increasing solution pH levels as the reaction proceeded. From both kinetic and thermodynamic perspectives, our findings demonstrate that high NH₃ production activity is characteristic of Mo-S-Fe₃O₄@FF—indicating significant enhancements in conversion rates from NO₃⁻→NO₂⁻→NH₃. Theoretical calculation, in conjunction with experimental analyses, reveal that the co-modification of Fe₃O₄ with Mo and S enhances electron transfer capacity during NO₃⁻RR. This modification forms Lewis-acid (Fe-Mo) pairs that increase nitrate adsorption energy alongside Lewis-base (S-O) sites that attract protons, thereby improving overall NO₃⁻RR activity. This report elucidates the synergistic effects of Lewis-acid and Lewis-base sites on optimizing catalytic performance for NO₃⁻RR using foam Fe-based electrodes.