Sulfur-Bridged Iron and Molybdenum Catalysts for Electrocatalytic Ammonia Synthesis

Carbon zero electrocatalytic nitrogen reduction reaction (NRR), converting N-2 to NH3 under ambient temperature and pressure, offers a sustainable alternative to the energy-intensive Haber-Bosch process. Nevertheless, NRR still faces major challenges due to direct dissociation of the strong N equivalent to N triple bond, poor selectivity, as well as other issues related to the inadequate adsorption, activation and protonation of N-2. In nature’s nitrogen fixation, microorganisms are able to convert N-2 to ammonia at ambient temperature and pressure, and in aqueous environment, thanks to the nitrogenase enzymes. The core NRR performance is achieved with sulfur-rich Fe transition metal clusters as active site cofactors to capture and reduce N-2, with optimum performance found for Fe-Mo clusters. Because of this reason, artificial analogs in Fe-Mo coordination chemistry have been explored. However, the studies of sulfur coordinated Fe, Mo catalysts for electrocatalytic ammonia synthesis are scarce. In this review, the recent progress of Fe-Mo sulfur-bridged catalysts (including sulfur-coordinated single-site catalysts in carbon frameworks and MoS2-based catalysts) and their activities for the ammonia synthesis from nitrate reduction reaction (NO3-RR) and nitrogen reduction reaction (NRR) are summarized. Further existing challenges and future perspectives are also discussed.