Orthogonalizing metal mediators and catalytic sites for high rate ammonia synthesis

Ammonia is among the most important industrial chemicals in the world today. It is used to manufacture the fertilizers that sustain much of global food production, and it can also serve as a carrier of stored energy. Almost all ammonia is currently produced through the Haber-Bosch process, which demands high temperatures (350–500°C) and pressures (150–200 bar). These conditions restrict production to a small number of large, centralized facilities that consume enormous amounts of energy. This leaves the ammonia supply, and the food supply that depends upon it, vulnerable to disruptions in energy and transportation. This project investigates an alternative approach that would produce ammonia from air, water, and electricity at ambient temperature and pressure. Such a process could be constructed in small, modular units near the users who depend on it, offering a cleaner and more flexible pathway to manufacture a chemical vital to U.S. food security and supply chain resilience. Among electrochemical methods for ammonia synthesis, lithium-mediated nitrogen reduction has demonstrated the highest Faradaic efficiencies and current densities. Lithium ions are electrochemically reduced to lithium metal, which spontaneously reacts with nitrogen gas to form lithium nitride. Reaction of the lithium nitride with protons yields ammonia. Despite numerous advances, the economic viability of this reaction is constrained by the high cost and limited availability of lithium. To address this limitation, an alternative strategy seeks to replace lithium with more abundant mediators such as sodium or potassium. However, these metals do not readily form nitrides at ambient conditions, presenting a fundamental challenge to their application in nitrogen fixation. To circumvent this limitation, this project orthogonalizes the roles of the metal mediator and the catalytic site, where these functions separated to distinct molecular species. In this system, the alkali metal functions exclusively as an electron donor, while a separate catalyst, such as titanium, facilitates dinitrogen activation. This strategy aims to preserve high nitrogen reduction rates while significantly reducing material costs and improving the overall energy efficiency of electrochemical ammonia synthesis. The research is organized around three aims. The first aim tests the central hypothesis that breakage of the nitrogen triple bond at a titanium(II) center is the rate-determining step. The second aim seeks stable, long-term operation by resolving how electrolyte composition governs the formation of the solid electrolyte interphase so as to enhance the transport and reaction kinetics of the titanium mediator and nitrogen gas, thereby improving ammonia selectivity and production rate. The third aim systematically evaluates a broader range of reductants and titanium complexes to further improve energy efficiency, rate, and selectivity. Together, these aims are expected to establish the scientific principles needed for a lower-cost, more energy-efficient, and modular route to ammonia. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.